We had a medium sized tree that was nearly blown over during the 80's hurricane and with some propping up and a few ropes we managed to save it. I suppose it depends on how much damage has been done to the roots.

Lilac can also stand being cut back hard and will regrow but, as Cammomile says, it depends how much root damage there is.

I would stand it up,stake it and give it a very hard prune.

Lilacs are very tough. there are no guarantees with living things but I'd be surprised if it didn't recover. It won't flower this year though.

I have often seen them recover from this in northern Arizona, but expect a flurry of suckers, and sparse bloom for the next year or two.