What Causes Difficulty In Walking After A Knee Injury?

In august 2015, I had MRI and the results show I had a torn minuscus. Walking on level surface was not a problem but I could not step up or down the stairs. SomehoW over time, about 6 months, my knee felt better up to the point I could do some low impact aerobic in may 2016. Then in august I felt a SNAP IN the back of my knee, I could not put that foot down for 2 days. Came the 3rd day, i could walk again but was not able to walk up or down the stairs. It got a little better over time again. But it seems it is taking longer and now it hurts when walking on a level surface. It feels warm and hurts while I am sleeping if I am not in the right position. It does not hurt when I touch it but walking up and down stairs is not possible.