I'm sorry but the above answer is incorrect. The microswitch that the parcel shelf sits on tells the car that there is nothing big enough in the boot to inhibit its operation.
What you need to do now is........
Inside the boot on the left hand side is a cover, remove this. Behind it you will see the valve block for the pump. Facing you on this valve block is a small allen key screw. Backing this screw off a couple of turns will release the pressure in the hood and you will be able to move it manually. Beware- if you are not confident with what you are doing do not attempt to carry out any repairs on your hood, it could literally cost you thousands.
Rich.
I would strongly advise that you take it to at least a specialist, someone who is familiar with how they work. But i'm afraid your warranty will only cover you at a BMW franchise as far i know. We're seeing Mini roof failures all the time now and it's quite an expensive affair when they do go wrong. Chances are good that if you did get to your dealer and the roof was found to be faulty they would apply for a goodwill claim as your vehicle is only just out of warranty.
From what you describe it does sound like a pressure build up in the valve block that is blocking the operation, which is a common occurrence. Have you managed to get it moving? If not then try disconnecting the battery for 10 minutes and retrying.
If the roof begins to move in a non uniform manner cease operation, bending the roof mechanism is generally terminal.
BMW Technician
5 years main dealer experience.
No problem at all, I hope it's resolved quickly and painlessly for you.