sun roof won't open it just makes noise

I just bought a 2003 Mercedes C320 coupe. the panormic roof blinds open and close but I can't get the Sun roof to open. I hold the button all the way untill the blinds fully open. Then I attempt to open the roof and just hear a thumping noise coming from the roof. The blinds open and close normally I think. They open in stages. First they move open 2-3 inches and stop. then I hit the open button again and they move another 2-3 inches. I repeat this until they are fully open. Then when I use this same button to open the roof I just hear a low "Clump!!". I must say that the owners manual is 'very' vague about most of the features on this car.
Any suggestions would be greatly appreciated!!

Best regards,

William W