I have an SLK hardtop (1998) but I can't get the hardtop to release... although the windows go down, the trunk opens, I don't hear the usual release of the hardtop. It was working earlier today..

 

Remove the small panel on passenger side of trunk - check the fluid level in the reservoir. I would bet it is low, not between the level marks on the side of the round tank.

When it is low there is a good chance it cannot complete the cycle and will present your symptoms.

Good luck.