Just been working on my drivers door release so a few ideas...
The cable may have stretched (if it hasn't snapped as suggested by Stunned Monkey).
The cable connects to the release lever on what looks to me like quite a weak bit of plastic - maybe this has snapped? Easy to check - one screw loosens the lever and the handbrake housing slips over it easily.
The cable restraints that stop the whole cable and sheath moving may have broken. One at each end.
The door release mechanism itself may be faulty - you'll have to take the trim off to get to that. A bit fiddly to get the mechanism out but quite an easy job all the same. You might find what's wrong with the electric release at the same time.
The whole mechanism is quite simple - good luck