"keeping it moist and in the sun"

Make sure that it does not get too much sun (morning only if you can help it; a shaded but bright spot otherwise) and make sure it is not planted deeply. Also make sure that the a/c heater vents are not throwing dry air on the plant all day and that water exit holes at the bottom of the pot are not obstructed. Mulch if not using mulch already. Consider covering/protecting the pot itself from sunshine if you notice that the outer surface of the pot is waaay too hot to the touch of your hand; some people place the pots inside another type of pot too.

Watering too much causes root rot (in hydrangeas and other plants). The signs of root rot are the same as those of a plant that does not get enough moisture so it is hard to diagnose an exact cause via the Internet. So do n-o-t water if the soil is already moist.

I should note that many newly planted hydrangeas will wilt (leaves and/or blooms) on their first or first two years in the garden. Once they get established then they do this less. What you will notice when this happens outside is that they recouperate on their own by nightfall or by early morning. So water only when the soil feels almost dry or dry to a depth of 4". Since they are pots, check often as potted plants tend to dry out more quickly than a plant in the ground.

Thanks, Luis. It's a new plant from the store, only 2 blooms and less than a foot tall -- the pot is the plastic one in which it came from the store. It's only potted because the weather is being uncooperative and I don't dare plant it in the ground yet for another week or two.

So, I will not water it again as long as the soil feels damp, even if it wilts on me again (it has perked up since being re-watered this morning and moved away from the cold windows into the sunny room.

I say "sunny" but really I should say "light-filled." That room has a mid-sized picture window -- still covered in plastic, with the shade down and the thin, white floral cotton curtains pulled shut. In the mornings it gets plenty of light, but no direct sun through all those layers. It's only about 58 in my house the past few days, so I don't think I am going to boil or fry the plant in there. I have it sitting on top of my pellet stove -- which is OFF for the season, so no heat, just a nice distance from the windows -- not too close, not too far.

I'll also check the bottom for drain-ability.

It seems to prefer it in there -- it's perked right up. So, I'll leave it there until it's safe to plant it in the garden outside, only water when no longer damp, not let direct sun get on it, and make sure it is well-drained.

Cross fingers for me!

I am having the same problem. I just repotted my hydrangea with a bigger pot. I cannot plant it outside until Mother's Day. I think I am in Zone 7. Western N.C. Should I cut the wilted parts off? Or just move it around the house?