Some actually do lock, I know there are some old Compaqs that can because I spent about half an hour trying to open one and then I found out it was locked in the BIOS
Try going into the BIOS (del or F1 or F2 or F10 or possibly another key) at startup and look through the settings, see if there's a case lock option
Unlock it from there and wait for the case to *click* before exiting