If you look in the owners manual you will find instructions on how to open the boot manually. It involves putting the seats down and reaching in to a small hole by the lock, pushing in a thin screwdriver or drill bit and the boot will open.
As far as why - it is probably a broken wire in the loom where it enters the body from the tailgate. The wires here bend and eventually break. The solution is to cut out a length of wire and solder in new pieces (warning they are all grey!) or replacing the rear tailgate loom (expensive)
If it were only the boot switch the micro switch at the boot opening lever can sometimes fail.
Do your rear wiper still work? That will often fail around the same time due to broken wires.
Citroens since 1969
Currently DS4 2.0hdi 180bhp EAT6