On a cabriolet the roof, the boot and the rear window are all connected. So if your boot won't open as usual you might need to look deeper. Is the heated rear window not working as well? Is the roof making a whirring noise at the end of the close cycle?
If there has been an electrical disconnection, battery, for example then the roof needs reinitialising before the boot and the rear window can work. If your radio has needed to have its code put in you will know that this has happened.
This has just happened to me and I found instructions on how to fix the problem in a post by loulou in 2011.
To reinitialise.....keep the open button depressed until the roof opens fully and leave finger on button for two seconds, then, keep the close button depressed until the roof completes the fully closed position, keeping your finger on the button for an extra couple of seconds after the roof has closed. The roof will then go through the open and close procedure all on its own and as it closes you will hear a definite closure noise. The you will see two things, the heated rear window will work and the boot will open!

Thank you so much for this really helpful (amd clearly written) post. It works! Thanks again.?