Depending on what type of hydrangea and your climate the most likely cause is that late spring frosts kill off the flower buds. Another reason would be if you prune back your hydrangea at the wrong time of year or if you prune some hydrangeas too much you are cutting off the flower buds so no blooms. There are winter hardy hydrangeas called Hydrangea arborescens such as the Invincibell Spirit hydrangeas that can take full sun in zones 4 to 9 and will flower even if you prune them in the spring because they flower on new wood. The more traditional or mophead hydrangea flowers on two year old wood so spring pruning or spring frosts remove flower buds. The exception is the Endless Summer mophead hydrangea that can flower on both old and new wood. Try not pruning your hydrangea at all. If that does not give you blooms next year then replace it with a hardier type as you can blame the cold for lack of buds. Keep growing, Marianne Binetti