I believe the hand brake must be applied and the car in the Park position. The roofs are complicated on most new convertibles you will have to bring it to a place that specializes in convertible tops or a shop with knowledge of Jaguars.
There are many problems that can be making this occur. Firstly, you should make SURE that you are performing the proper steps required in your owner's manual to open an close your convertible top.
There are many likely problems:
1.) A fuse is blown (THE MOST LIKELY REASON)
2.) A safety mechanism is preventing the top from opening
3.) The procedure for opening the top is not being done correctly
4.) The actual switch, motor, or other component is not properly connected to the circuit
5.) There is a short in the circuit (this may be caused by water damage
That's a very ambiguous question. If you go to jaguarforums.com you can search do a search for problems with the convertible top system. There are a lot of members there who know a lot about Jags, and there are even a few jag technicians who hang out there.