Forsythia and other early flowering shrubs form their flower buds the summer before and they winter over on the plants.  Last fall and early March 2016 were very warm so the bids were advanced toward blooming much too early.  Then April 4th, 5th and 6th happened with lows of 18°,14° and 17°.  That froze the buds and so, no flowers this year.  Many magnolias and azaleas suffered the same fate.  The plants will be fine and leaf out just fine.  This happens every so often