Like any other moving parts the door lock cylinders, latches, and activating rods need to be lubricated occasionally. The lock cylinders should be lubricated with a lubricant that is dry and will not attract dirt, such as a silicone spray lubricant. The latches should be lubricated with a lubricant that will remain pliable in all temperatures such as "Red Grease", Lubriplate 1200-2, or white lithium grease.
None of the above will help you now. If you cannot open the door via the inside or outside handles you have to determine if it is the latch that is stuck or the lock mechanism. If it is the lock mechanism you may be able to get a local policeman, towing operator, or body shop to try and break into the car door with a "slim jim". This device is used to phisically move the actuating rod that trips the lock mechanism. This should then allow you to release the latch by the door handle.
If the latch mechanism is frozen then the door panel has to be removed. The easiest way to do this is to remove the seat so that more room for access is available. The one screw on the outer edge of the door (near the top) is almost impossible to get at but it can be done. You obviously cannot use a screwdriver but it can be grasped with a narrow needle nosed pliers, or vice grip, and turned out. It is not very long but there is barely enough room to get it out. Again, it can be done but requires patience and finesse.
The rest of the fasteners are easily accessed, but the push pin fasteners will be difficult to remove because of the lack of access to the edge of the panel for purposes of prying. The key word here is patience. If you try to force the issue you may damage the door panel. rangerj