Check that the door handle has returned fully forward under the action of the internal spring. If the doors are usually operated from the key-fob then over a period of time the outside mechanical handle can become stiff because of a build up of oxide on the hinge mechanism. This can cause the handle to stick in the 'partially open' position even after you have let go. To fix this, spray with WD40 penetrating oil from time to time to reduce the oxide build up.
The external handle should operate the latch at the back of the door but some have found that the bowden cable to the door latch can stick in the open position. Before too much dismantling try slackening the bolts holding the door lock in position - one at least has heard a "click" and all came well. A long shot but if you are lucky will save you a lot of pain!
OR - the cable could have snapped or come loose.
OR - the moving part of the external door handle (the part you pull) has a part attached on the inside that pulls the end of the bowden cable forwards to release the lock mechanism. This joint between the two parts of the handle can wear sufficiently so that moving the handle has little effect on the other part.
First remove the internal door card and the dust seal - see see here
Then you need to remove the window which is held on to the slider by clips that are very difficult to release. The window is easier to remove if you have a companion: lower the window to mid position, then one person releases the clips while the other person grabs and pulls the glass up. Releasing the clips is the hard part, don't be tempted to use a screwdriver - only fingers or you'll break the window or clips.
After the window is out you need to remove the motor and rails before you can get the door handle out. It is not a very easy or fast fix.
Repeat: the glass fixings inside the door holding the glass to the runners are a push fit but most monkey mechanics pull too hard and they snap. The clips are white plastic and fixed to the runners. The glass has a hole either side and simply pushes into the clips. Getting them out is when they break.
Best way to fix is to take the door off - lay it flat on the floor and strip it out there. Parts are available. There are procedures to set the glass in a certain position first but if you're doing this just ask Clive Griffiths.
However Justin suggests: removing and refitting window glass is an easy one man job. Just use a medium to small flat blade screwdriver to carefully push the plastic lugs and pull one side of glass up at a time. Remove the outer door glass rubbing strip to make it easier.
After you remove the inner door trim, you should see 4 holes covered with foam. Uncover these and with a 10mm socket spanner loosen the four screws and remove the handle. BEWARE: you also need to remove the window (see above) because one bolt will be obstructed by the window.