Some cases used to have a key in the front that was connected to a jumper on the motherboard, if the key was unlocked, you could use the computer. If the key was locked then you would get the error message you are getting. If you have lost the key for the front of the case, you'll have to remove the connector from the "keylock" jumper on the motherboard. If the case has no key, then something else may be incorrectly hooked up to the "keylock" jumper. If nothing is connected to the jumper, then you may have a freak problem with your motherboard.
Zenmervolt