Is it plugged in? Someone might have disconnected the wiring harness. Are you using a valet key? Check the manual for a valet function.
Check for power at at the trunk lock solenoid, the switch, and the fuse with a multimeter or test light. Work backwards until you find the reason for the no voltage condition.
No need to shout, Satch; you're impossible to ignore in any situation. To be (more) clear: Have you (the OP) somehow gotten the trunk into valet mode? Perhaps by accidently double-locking the trunk? Or is there a valet lockout switch in the glove box (there is on my newer 335d)? Never having looked into an E39's glovebox, I personally have no idea if there is one there.
