stla,
Hard to answer since there is no model # or description attached (laptop??? desktop???) but here goes.
I am assuming you mean that when the side panel of the desktop is opened while the PC is running (although I don't know why you would need to do this). There is a setting in the BIOS to enable/disable case tampering, usually called Chassis Intrusion or Case Open or something similar.
Some corporate PCs have this feature enabled so even when the PC is off the case cannot be opened without the Admin knowing about it. In this case there is a little switch near the chassis door.
If you get a "Case Open" warning upon boot you will need to clear the CMOS Case Open Log by resetting CMOS (best) or removing the battery for a few minutes. There should still be an option in the BIOS to turn the feature off.
Tony
"...an eye for an eye leaves everybody blind" - M.L. King