You've probably pulled the fibre optic away from the LED's in the head.

Strip down the fibre optic cable entry on the top of the head and give it a clean. You'll need two spanners to undo the gland, so that you don't twist the cable in the head. When you reassemble, note that the spreader inside only goes back in one orientation, but that you can get the cable in one of four positions. Trial and error will see you in the correct one. You can trim back the ends of the optic cable if you wish, it's only polymer. Just use a sharp knife.

The HUD end strips just as easily.

I managed to screw mine up inside a wreck in the Solomons. 10 minutes with some spanners sorted it out. That was AFTER the dive, before some pedant says anything!