Redownloading Game After Computer Restart

My computer been locking up while in champ select ( good pc) forcing me to hard restart, then when I open up league it gives me the" Welcome to League of Legends " login screen with no saved login credentials, which I always have check-marked. -Also sometimes when I'm in game It starts to massively slow down going dropping massive frames ( like 2 from 170 FPS) accumulating to the computer locking up ( It doesn't force me to patch like other times when this happens)