Suggest Cure For Pain Behind Knee

I've had in the past that my left knee would "give out" but then it was fine for last 9 months or so. Just recently I have excruciating pain behind my knee around to the front. It feels thick when I try to bend it and I can't straighten it without pain. Also obviously have to walk with a limp because I can't extend it fully without pain. No injuries as I can remember. It comes and goes.