Electric boot release, while the switch or the release solenoid might have failed, the most common cause of problems is one or more broken wires in the harness where it jumps from the body to the tailgate or boot lid.
The last time I tackled one of those I was fortunate enough for it to be on an estate model so it wasn't too difficult to get inside, remove the trim and using a slave battery and a couple of jumper leads to energise the lock.