If it won't open with the remote or with the button on the dashboard it has to be an electrical problem. If you can get to it try cleaning the electrical contacts on the release mechanism. If that fails it could be that the unit has failed and will need replacing.
I had a similar problem many years ago on a Vauxhall Cavalier when one door wouldn't unlock. I swapped the motors on the two back doors over to see if the problem lay in the motor or the wiring to the door; the problem went away. Makes me think a mucky connection is the most likely problem.