I have experience in this. You need new yeast, almost definitely (depending a lot on what strain you used, how long you fermented for, and if you did a secondary) but almost definitely yes. I would rehydrate about a quarter of a packet of champagne yeast. Put it in the bucket along with your cooled priming solution, rack the beer in. You might not NEED to do this, but it will certainly save you the heartache of having a beer not carb.