I'd suggest you get in touch with the ward where you were discharged from. If they've cocked it up, they should put it right. But knowing how hospitals dump work on GPs, they'll probably get you to see your GP who may or may not need to speak to the on-call orthopaedic team to get you seen. Alternatively, if they're into minor surgery, they may put in a stitch or two to hold things.
What kind of knee op did you have?