These guns have a key switch underneath, has it moved? Have you tried making sure it's turned on with the key - turn it off/on again?
As it's not a display version it's easy to get "lost" in the program is you don't know what it's doing, and some parts of the 10 year old original instructions were incorrect - I wrote these for myself to keep me right on non IS Airwolves/MK4s
It may be you have the mag counter set and it thinks it's empty or something silly like that.
Hope this helps in some way.
A guide to the Mk4 MCT settings:
1 Bleep Resets the shot counter for the magazine
2 Bleeps Turns the safety catch light on and off - 1 bleep off, 2 bleeps on
3 Bleeps Turns the magazine shot counter on/off - 1 bleep on, 2 bleeps off
4 Bleeps Low pressure warning. The warning can be set from 50BAR+. each trigger pull adds 10BAR to the level at which the warning goes off
5 Bleeps Power adjustment. Each trigger pull reduces the power 1 stage of 16 increments
6 Bleeps Single shot mode
7 Beeps Reset to factory defaults
James