If the top lifts up a little but does not disengage from the pin at the rear, it is probably due to a bad cable adjustment.
The small lever inside the car should pull a locking pin to the right. Once you get the top off, you will see how simple it is.
When I had trouble with mine, I made a little tool from a bicycle spoke. A coat hanger would do as well. Just bend it into an "L" shape, with the "L" about an inch long.
Lift the hardtop a fraction and support it. Then insert the short end of the tool into right hand side of the hole. You need to push the locking pin a little to the right. Should only take a few seconds, and voila - the hardtop is free to lift.
Previous cars of interest
1976 Jaguar XJS
1974 Rover 3500
1953 Riley (before they became scarce)