On my home computer (that has MS Windows XP) I have accidentally pressed some key combination (what it might be?) that locked the computer. The lock screen displays current user, and number of running applications, but asks for a password.
This computer is set up to automatically login without password (I don't think that password is even set, and if it was I don't remember it). Can I go back to my running application (with unsaved data), or do I to come to terms with the loss of data?
Have you tried just hitting [Enter]? Your password could be blank. If not I don't believe there's a way to do this that doesn't involve forensically capturing data stored in RAM, which involves other tools and software.
If you've tried obvious passwords like and "password", and exhausted your mental list of what you think it might be, then accept the loss and reboot.
Otherwise you're up against one of the main security features of the OS, which won't be easy to defeat (if it's even at all possible in the current state).
After you reboot and it automatically logs back in, check the auto-login registry entry to find out what the password is set to (it'll be in plain-text) -- for next time. :)
