Without a photo or two or three any diagnosis or attepmt to help is going to be very iffy.

All rhododendrons naturally shed leaves after 1 to 3 years. Before falling they turn yellowish and frequently have spots as do some of the less mature leaves, so the initial pruning in April may have been unnecessary.

Overall sickliness in rhododendrons and most other plants is do to the soil's fertility problems and/or physical condition. A soil test by county extension service, etc. is a very good idea.

It's highly unlikely you've killed the rhododendrons - have the soil tested, follow the recommendations from the soil test, and be patient. Don't prune any more this year. Late growth, should there be some, will not have enough time to harden off before winter.

There are many good books on rhododendron culture which will help you much more than a brief post. Rhodyman's website is also very good.

Aka is right, pruning probably didn't do any harm. If you mean by "branches not green at all" that there was no green cambium layer under the bark, then that branch was dead and your pruning did no harm. If you mean that there were no green leaves, then you probably cut off live wood.

1) The chief cause of rhododendron death is water, either too much or not enough. Too much causes root rot which is terminal. Too little causes dieback which kills the plant one branch at a time. Rhododendrons and most other plants like "moist well-drained soil". That make it sound like no matter what you do you can't do it right. However, if the area has good drainage and you use mulch, it should be easy to achieve moist well-drained soil. To check for good drainage, dig a hole about 10 to 12 inches deep and fill it with water. Then after it drains, fill it again and see how long it takes to drain. If the hole drains within an hour you have good drainage. If the water has not drained out of the hole within one hour, the soil is poorly drained and you must correct the drainage problem before planting. Install a perforated pipe or drain tile in the garden, making sure that the outlet is lower than the bottom of the planting hole, or build raised beds. If the drainage is OK, then you can keep the area moist with a good mulch layer. Mulches conserve water in the soil, insulate roots against summer heat and winter cold, and discourage weeds. Replenish mulches annually, as needed, to maintain a 3- to 5-inch layer on the soil surface. Fine-textured organic mulches such as pine straw or shredded bark are best. Fall leaves are an excellent mulch. During hot dry weather, it is common to see rhododendron leaves look wilted. It they look wilted in the heat of the day, that is normal. If they look wilted in the morning, that is a sign that the plant is either too dry or dying from being too wet. It should be easy to tell which, but don't assume, check the soil with your finger. During drought periods, it may be necessary to water once in a while. A deep watering once or twice a week is much better than frequent watering. Only water when the plants show signs of being dry.

2) If the soil is well drained, then the second most common problem is improper planting. Since your plants were already planted and may have been planted a number of years ago, this is a hard area for you to consider. However, if you find a plant has not put out new green shoots in a year, it is dead and you can dig it up and look at the roots. If they are growing in a circle and strangling each other, the plant was not planted properly. You could dig up the other plants and try to open up the roots so they don't strangle each other. If necessary, cut some of the roots so they don't strangle others. If you do dig them up, never let the roots dry out. Dip them in muddy water occasionally while working on them. If they dry out, they will die.

3) The third most common cause of decline and death is improper nutrients and pH. Rhododendrons need acidic soil, a pH of between 4.5 and 6. Fortunately, rhododendrons are great pH detectors. If the leaf is green, don't worry. If the leaf is yellow with green veins, you may have a pH problem, however it could be a nutrient problem also. In any case it is chlorotic. There are many causes of chlorosis. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency or too much phosphorus in the soil. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form, due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways. Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. For a quick but only temporary improvement in the appearance of the foliage, ferrous sulfate can be dissolved in water (1 ounce in 2 gallons of water) and sprinkled on the foliage. Some garden centers sell chelated iron that provides the same results. Follow the label recommendations for mixing and applying chelated iron. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer. In any case, never use aluminum sulfate. Although garden centers sell it and it is great for hydrangeas, it will kill rhododendrons and azaleas if used repeatedly. Also never use fertilizers with chemical nitrogen. Always use a good rhododendron and azalea fertilizer with organic nitrogen like HollyTone. Also, always fertilize in the spring and at half the rate on the package.

4) The last most common cause of decline and death is insects and fungus, things that you can spray for. The most common causes are cultural, things that arise because of where and how the plant was planted.

Here is a link that might be useful: How to grow rhododendrons and azaleas