What is this?
Lilacs bloom in the spring, usually mid-May, so that isn't likely. Your leaves look like a type of hydrangea that can be pruned to tree form, called hydrangea paniculata. I can't the exact variety without a flower, but pee gee is one that is very common. They also usually come back if cut back severely, which is what you have experienced. There are many new paniculata varieties available on the market, Limelight is quite popular. Here's a link to more information: http://www.hydrangeashydrangeas.com/paniculata.html