Don't put more dirt over it......smothering the crown could produce rot not to mention piing excess soil over the roots can deprive them of needed oxygen. If anything, you could pile some loose mulch around the base - but not over it - although it is probably too late in the season for that to be of any value.

Although many types of hydrangeas do look like a collection of dead sticks in winter, most will come back to life as temperatures warm. Some may not, depending on the type and your location :-))

btw, pruning back is also very dependent on type. What kind of hydrangea are we talking about and where, generally, do you live?

I would maintain the soil as evenly moist as you can and keep the area well mulched. By the end of May, I would prune any stems that have not leafed out. If it made it thru winter, it should either leaf out or show new growth from the base/crown by the end of May or thereabouts. The condition of the root system (alive or not) is what is important. If the roots survived winter, it may develop new growth or leaf out if the stems are ok inside.