I have had success with a mophead that dried out during the summer and leafed out again in Spring. I did not extract it but you can replant it once temps have moderated and Fall weather has arrived. It will be sensitive in future summers to water issues again but eventually, these problems should diminish. Just mulch it well when you replant it and maintain the soil as evenly moist as you can either with rain or sprinklers.

Fall is a good time to plant hydrangeas. Or you can put it in a pot/container, store it in a shed or garage, water it every 1-2 weeks and plant it in the Spring.

shebb, Keep it in the bucket of water! It will revive - some will warn you not too, but keep it in the water!! Can't tell you how many just like this and a lot worse I have revived. Don't worry about the 'best time of the year' this will work. Put it in the shade - no sun for now and leave it alone. Check to make sure there is water in the bucket and when it cools off you can plant it in a container so it can be watched. The leaves should perk up and it will continue to thrive. Hydrangeas - especially older varieties are really tough. When it looks better cut three or four branches and put them in a pot of moist soil. Cut the branches - about 12 to 15 inches long - just above a leaf node - the place where the leaves attach to the branch. Pull - do not cut - the leaves from the branch leaving only a few at the top - 3-4 will do. Make sure there are several nodes buried in the soil. Put these in a shady spot and keep moist. They might start to wilt and look terrible, but have heart and watch closely and new growth will eventually start from the nodes that used to be leaves.

The trick to making successful cuttings is to fill the pot or container completely with any potting soil. Don't waste your money on anything fancy or organic - garden soil will also work. Do not put rocks or screens or drainage stuff at the bottom of the container. Keep the soil moist and give them time.

In a week or two or even three, take your large hydrangea out of the bucket and plant in a container adding the soil in the same way as above. I would keep the plant in the container till spring and then replant in the garden. It should be just fine and you may have a few nice cutting rooted to share with others in the family. I noticed that you are in zone 4 - so it might be a good idea to winter over your rescue plant in the garage for the first winter.

I wouldn't keep it in water for more than a day or two to let it rehydrate. After that, plant it in a pot of potting soil with a saucer under it to keep the soil from drying out, and plant it in the ground in late September or whenever we start getting regular rain, but not later than mid-October so that it has time to get established before winter. Mulch well. The reason to not let it sit in water long-term is that most plant roots need oxygen and standing water or fully saturated soil will kill the roots by preventing air exchange. You can try cuttings as well as ophoenix suggested, but IME hydrangeas can often be revived if well hydrated and replanted.