I hit my knee bad on the floor 5 month ago and now i hear crackling sound on my knee whenever i get up or going up in the stairs...what's wrong?

Chondromalacia: There are several possibilities that could explain what is going on.These include arthritis, patellar chondromalacia,patellar tendonitis/tilt &/or patellofemoral syndrome alone or in combination. Thus it is best to see a Sport medicine or PMR specialist for a thorough evaluation X-rays and referral to PT for quad/core training. In the meantime glucosamine or natural anti-inflammatories as needed.

Let me try to help: when you hit your knee, injury t the cartilage which cover the bone happened, that lead to some and fissuring to the cartilage surface which lead to the noise and cracking.