I don't know anyone who hasn't fallen off the wagon at least once (at least) while trying to lose weight. So you're not alone! If your knee is still busted or is healing, you should talk to a physical therapist about what kinds of exercises you can do at the moment. If you're healed but still rusty from not exercising in a while, take it very slowly! Be very careful. Yeah, your weight loss is important, but so is your knee, which is a very easy thing to ruin.
And my advice about the scale would be to stay off of it until you know that checking it won't make you anxious or depressed or unmotivated. Make healthy food choices and do what you can, physically, and return to the scale when you've forgiven yourself for falling off the wagon.
Just remember that it happens to almost everyone, no matter the amount we're each trying to lose.