game screen will come up, but mouse and keyboard will not interact with the game

Just happened, I alt-tabbed out of the loading screen, as I almost always do, alt-tabbed back in after I heard the game start. however, my mouse cursor wouldn't change from the regular, windows mouse to the in-game pointer, nor could I press P for shop, or chat with allies, or anything. none of the keyboard buttons worked other than ctrl-alt-del. I reconnected at least once, then exited league client and reconnected, and the same thing happened. I had to restart my whole computer to get it to work again.