These are not plants that can be divided, they're shrubs. Assuming its in the ground, have conditions changed around it, ie maybe a tree or other shrub nearby has got much larger and is leaving the hydrangea deficient in water and nutrients? Have you added any organic mulch or garden compost, or even better, composted animal manure around the roots in recent years, or has it been left entirely to its own devices? Do you prune it at all, and how old is it, or rather, how long have you had it?