Well what do you mean exactly by you can't fold your knee. As in retracting your leg back so that your ankle/calves touch your quadraceps? Additionally, are you just saying you cannot literally fold it, or you can, but its painful? What exactly was this accident and where precisely did they do the surgery on, and for what purpose? For something as serious as that, I'm not sure what you are doing on a health forum with people who don't know that much more about their bodies then you probably do, when you should check it out with doctors to see if the surgery has gone wrong or failed, or if its a side effect, etc.

after my surgery about a year ago for my knee i had some complications afterwards and so was not able to get full motion back in my knee. i had probably only around 60 degrees or so at the time so my doctor did a manipulation (put me under general anesthesia then gently but repeatedly bent my knee so that eventually he was able to get full range for me). the manipulation was able to get rid of a lot of adhesions that were in my knee (which was part of the reason why i wasnt able to get my full range of motion back) so you may want to discuss this option with your doctor. also, if youre just looking for some tricks you can do by yourself (even though im sure you probably know the same ones i do from PT and stuff but..) you can always try heating your quad and/or hamstring before/while doing range of motion excercises. you can also try bending in different postitions (your side, your stomach, your back, on a chair, on a bike, etc.) hope this was at least a little helpful and hope you get your motion back soon.