Minecraft says 'Game already running' when I start

In the launcher, when I press play to start Minecraft, a popup window comes up saying 'Game already running'; 'Already running in this directory' and 'Already joined the game as this user'. Minecraft Launcher was the only thing open, and I restarted my PC a couple of times, to no avail. When I press 'Continue anyway' I am unable to join any servers and it tries to connect but an 'Invalid session' error shows up. This means I am only able to play singleplayer. And by the way, I don't know what the versions in 'Affects Version/s' mean. so I selected all of them.