Did you move the pot to a different location? Otherwise, cut it back, and use a fertilizer high in phosphorus content next season. That should help it bloom on the new growth next year. Finally you might try removing it from the pot and trimming back the root ball a bit if it appears to be root bound. I had a hydrangia in the front of my home that almost never bloomed (not in a pot though) and frustrated I cut the whole thing down removed the roots and threw the whole thing over the back garden fence into the woods. The next year I found that hydrangia as big as it was when I took it down, growing where I tossed it with huge flower heads. I'm still trying to figure that one out though I strongly suspect it was giving me the finger for ripping it out and tossing it away.

Did you prune it? Hydrangeas flower on the previous year's wood so pruning can cut out next year's flowers.

That's right - Wait until spring and then cut off last years dead flower heads only. If you need to reduce the size don't cut it all back at once - that way you will still have some stems that will flower. But bear in mind that the mop headed ones are different from the paniculatas which have flowers shaped more like lilac flowers - they flower on the new growth and can be cut hard back.