Can't straighten knee without pain?

I had this exact issue after a football match where I couldn't lock my knee straight. Turned out to be a meniscus tear and I had to get an MRI done. Don't try to stretch it forcefully, just keep it bent and go see a sports doctor ASAP.

Listen to me, don't try to walk or push through the pain. Keep a pillow under your knee for support so it stays in a comfortable position. We will get an orthopedic doctor to look at it tomorrow, just rest for today.

If you cannot straighten your knee, it might be due to a ligament issue or fluid accumulation inside the joint. Do not apply hot fermentation if there is swelling, use an ice pack instead. Please get an orthopedic evaluation as soon as possible.

In our days, we used to apply a warm poultice of turmeric and salt to reduce joint swelling. It can give some mild relief, but if the knee is locked and cannot straighten, you must not ignore it. Please visit an experienced doctor for a physical checkup.

I panicked completely when my knee locked up after a gym session, thinking I needed major surgery. But it was just a minor hamstring tendon strain and some fluid buildup that resolved with two weeks of physiotherapy. Go to a doc, it is usually not as scary as it feels.