The hobblit

So when I was out this morning I had a little twinge on my right knee. So I stopped running for a bit. Felt the same as what I did to my left knee a few years back , and now I can't walk up of down stairs. It's defiantly the same injury but on the other knee, so looks like I have to lay off the running for awhile . Boohoo. Any ideas what it might be ?