To unlock your computer using the UNLOCK COMPUTER dialog box, in the Unlock Computer dialog box, type your password.
To unlock your computer using the COMPUTER LOCKED dialog box, press CTRL+ALT+DELETE, type your password, and then click OK.
Found via windows help and support under lock computer.
Possible things that could cause it; someone hitting the LOGO + L key combination.
To open Display, click Start, click Control Panel, and then double-click Display.
Selecting the On Resume, password protect check box will LOCK YOUR COMPUTER when the screen saver is activated. When you begin working again you will be prompted to type your password to unlock it.