The lock will be full of crap so the key does not work. Try to spray so oil or something and try to free it.
9 times out of ten the reason for the boot not working off the central locking is the bar that actuates the lock/unlock is rusty and tight. Best thing to do is pop the rear seat down and remove the plastic trim off the boot (5 trim fixings hold it on). Once the trim os off you can pop the boot manualy by pulling the lever to the catch upwards.
Once the boot is open and now with it up look at the centre. There will be a long thin bar that goes to the blue actuator on the right. Try to move the rod left and right as it will be tight. You will normaly see a rusty section of the bar just to the right of the lock and this is the part that is the problem. Spray it with grease and try to work it loose. Once loose close the boot and test.
