Unless this car is very unusual there are only two places that can cause this to happen. Both are inside the door panel. The handle normally has a screw holding the backplate (cosmetic covering so you don't see the inside of the door) to the sheetmetal of the door. There is a small rod approximately the diameter of a wire coat hanger that connects the back of the door handle to the lock mechanism where the door latches. This rod must pivot at two points for it to work and at each of these two points there is a clip that holds the little rod to the lever on the handle or latch. What happens most of the time is the clip will fall off (or rotate around enough) and the rod will fall out of the back of the handle or latch. All you have to do is to reinsert the rod and snap the clip back in place.

Now if it is a power lock issue the same thing could be happening but instead of the door handle it could be the power lock solenoid. Your question could be read two different ways so if the handle moved easily but nothing happened or if there seemed to be some resistance when pulled and the door wouldn't open. No resistance would mean the door handle to latch route. Resistance would mean the solenoid or something else was blocking the lock from unlocking. Don't use WD-40 on locks inside the door because it seems to "attract" dust and can cause more problems than it solves. Use graphite powder in door locks and lock assemblies for the best results.

Good luck,

Kaiser Willy