Usually tearing a ligament in your knee will cause a popping or even a snapping feeling at the time of the injury.
It should be able to be heard or felt if the ligament is torn completely. Your knee may also swell up and their might be bleeding inside your knee.
When you tear a ligament in your knee the best thing to do is go see a doctor for treatment but you should prop your knee up on a pillow at home when sitting or lying down.
Wearing a knee brace will also prevent further injury and stabilize your knee.
The torn ligament in your knee should fully heal within 6 weeks.