Sounds like Fast User Switching magically disabled itself. If you press Ctrl-Alt-Del, you'll probably see something like this: (not my screenshot, Windows won't let me do that)
You need to go to Control Panel -> User Accounts -> Change the way users log on or off
Then check the box marked "Use Fast User Switching" - make sure the Welcome Screen is enabled, otherwise you can't do it. Hit Apply Options, and you should be all set.