What caused the uprooting? Weather or an out of control vehicle or . . . ?

I would most likely do some pruning to remove damaged branches and make it less top heavy, sink some robust stakes into the ground on the uprooted side, and pull it back into place and tie it to the stakes using ties that are broad enough to not damage the bark and stems you are tying to, such as wide strips of old T-shirts or stockings. Make sure that the root ball on the uprooted side is well covered with soil and water it in. IME lilacs are quite resilient and it takes a lot to kill one. Mulch over the root ball to help keep moisture even! And keep an eye out on water this season all the way into fall and right up to the ground freezing so that it doesn't dry out, but don't keep it soggy. I would be surprised if it doesn't recover completely. By next spring I would loosen the ties so the plant can move to see if the new roots can support the plant. If you have a longer growing season than I do you may be able to do this sooner.

a pic or two would be worth a thousand more words ... plz ...

you can barely kill the things ...

after bloom.. if thats important ... i would probably heavily rejuvenate prune it ... and dig out and remove the bad parts.. and leaves the good parts there ...

or if it appears to be a total loss .. i would take a couple root cuttings... and start a new one ... something like this:

http://fivecedars.com/starting-and-growing-lilacs/

i would not waste my time.. trying to make the broken stuff stand upright .... just not worth it with a lilac ....

ken

https://duckduckgo.com/?q=rejuvenation+prune+lilacs&t=ffcm&ia=web