My long lilac hedge was planted in 1900 - so it's lived for 112 years so far.

Is yours in sun? They need full sun to really thrive and bloom. Any obvious signs of disease? Are you just trimming away the dead wood/branches or cutting it back to keep it at a certain size? Unless your soil is really bad or deficient, lilacs should grow like weeds in Wisconsin - like they do all over the midwest - without the need for fertilizers, etc. Without seeing it, I'd suggest leaving it alone for now and see how it leafs out in the spring.

if its in tree form.. compared to the above HEDGE..

i would suggest that a picture might get you relevant advice.. rather than pure speculation ...

otherwise google: rejuvenation pruning of a tree form lilac ...

ken