any mods installed? if so, uninstall them. Also try to reset your control settings for the game.
i have aslains modpack , with gunsounds garage mods and hd skins , it was fine last night but for some reason not today , ive tried in safe mod still doesn't work , so its not mod related. I'm just going to reinstall game , but will take around 16 hours to download. ive even removed the modpack still no joy , ive also tried clearing game cache still no joy