I Just Injured My Knee Playing Baseball. It Feels Like

I just injured my knee playing baseball. It feels like I overextended it backwards. Feels like I have a charlie horse just below my knee in the back. Also, both sides of my knee area hurt. I put full weight on my leg throwing a ball and my knee went out and I fell. I tried to get up 3 times and fell back down. Do I ice it?
Do I stay off of it? For how long? OUCH