Hydrangeas need well-drained soil to prosper.  All plant roots need moisture, but also air.  Overwatering, especially consistently, squeezes out the air and allows the roots to develop fungal root rot.  The question for your plant is has it developed root rot, if so how extensive is it, and bottom line - can it recover.  The first step, of course, is to stop watering.  What and how severe signs are your plant showing - wilting, yellowing leaves, shedding leaves, general unthriftiness?  How long have you had it?  If the plant had been around the store for awhile, it may have been root bound when you bought it.

 If your hydrangea is in a pot, remove it and examine the roots.  They should be whitish and firm, rather than brown/black and mushy.  If there are spots of rot, cut these away, carefully loosen the roots, wash the roots with a gentle stream of water, replace the soil with fresh potting soil and repot in a clean pot.  If your plant is in the ground dig down carefully a couple of inches and examine the roots.  If the roots look okay, it will probably recover.  If not, probably not.   Hydrangeas need extra water when first planted because the root system has not adjusted, but then should be watered deeply just once or twice a week.

This article, Hydrangea Questions and Answers, may give your more information on hydrangeas.