Okay, so I'm 56 years old but in good health. The usual achy joints that come with age, but nothing major. Yesterday I suddenly noticed my left knee won't bend! No significant pain. It feels swollen, but apparently not enough for most people to notice when they look at it. I haven't injured it that I know of. Has anybody else had a problem like this? I'm just wondering what is causing it, and if it will improve on its own.