Perhaps root rot.
If so, never plant another rhododendron in that part of the garden.

This past winter with low tempratures and high winds was very difficult for rhododendrons throughout New England. The leaves in the photo show typical damage from so-called winter burn. Nothing visible in the photo suggests any root rot problems. If this is a different variety than the ones planted nearby, it may simply have been a more sensitive type. Seemingly minor differences in exposure to winter sun and wind can also make a big difference.

The real question is what were the symptoms of being "not so healthy last fall"? If it was planted two years ago and failed to set any flower buds in its third season, I'd say the most likely problem is that the root ball was not sufficiently untangled when it was planted and has dried out and not extended its roots. This is a very common problem.

Miracle Grow is not a very useful fertilizer, especially for a stressed plant, because it does nothing to slowly feed over time. No fertilizer at all is a better idea until the cause of the plant's decline is determined.

I am absolutely agree with you that the root ball may not be big enough and its root was damaged. The symptoms of being "not so healthy last fall" were its leaves did not like others full of energy. I was always thinking it needed more water and I did gave it enough water.

Its original owner wants to get rid of it and its brother. We went to dig them up and actually took back more than 10 of them (big family with many "kids"). Most of them are doing well.

If it is caused by root is not big enough and damaged, can I cut most of its branches to save it?

we love rhododendron and we want to save it if we can.

Woody plants need branch tips to generate auxins which stimulate root growth.

If the problem is with the root ball, dig up the plant, untangle or cut if necessary the roots, soak in water for 30 minutes or so if it is very dry and replant. Peat is the chief ingredient in planting medium for pot grown rhododendrons. Once it dries out, it's very hard to rewet. Further, it shrinks when it dries. This leaves a gap between the original root ball and the surrounding soil that prevents capillary transfer of water. You need to amend the native soil so that it is at least similar to that of the rootball. Use bark, conifer needles,or any acidic organic matter, not more peat. Water this summer by directing a very slow trickle at the base of the rhododendron. The idea is to moisten the rootball without wetting the surrounding soil. You may have to water in this way a second year as well.

Cutting back branches may be of some very minor help in reducing transpiration from the leaves, but is probably not necessary. Rhododendrons can support an amazing amount of top growth from a very small root system as long as the roots don't dry out.