Help! MacBook Pro freezing

Hi guys, I'm not sure where to look.. And if this keeps up I'll go insane enough to quit.. Please help. I have a MacBook Pro 15", OS 10.9.5 (latest I think) I don't have other programs running in the background.. Have been playing league fine for ages.. However the last few days it's decided it likes to freeze up..

Questions for the intelligent people out there.. 1: why could this be happening? 2: how do I stop it?? 3; When it happens, how do I change Windows or force quit..?

Thanks in advance 🙃