Sounds like either the thread is all chewed up in the hole on the front panel, or else the screws are out, its just friction on the back panel stopping them falling out. To tell the difference, tighten them up again (clockwise) and see if they do come to a stop.

In both cases the solution is to gently attempt to prise the back off the front. The back has very little weight and the monitor is attached to the front panel, so put the screen downwards on a pillow and use a flat bladed implement to try and increase the gap between the casing halves.