This happened to me with my 2003 at 105,000 kms. A spring breaks in the lock mechanism and the mechanism will not unlock - the mechanism reacts like trying to unlock the lock while holding the handle open.
The door panel can be partially removed as flashstar mentioned. There is little to no space to work though. I managed to be able to remove the glass from the door, and from there I was able to see what I was doing from the gap above the door panel where the glass slides.
To get the door open:
Detach the metal rod that connects the door handle to the lock mechanism. There is a plastic clip that catches on the rod and keeps it from backing out from the hole. Rotate the clip until it unclips from the rod and then the rod should come free from the hole on the handle.
Lock the door.
Push the rod downwards. Twist hard, then pull back upwards. This should reset the lock.
Unlock the door.
Push the rod downwards. Hopefully the door will pop open, if not repeat and try again.
Once the door was open I removed the lock mechanism and dismantled it. I found a spring had broken at the end. I bent the last coil at the end of the spring to make a new hook, and hooked it back onto the mechanism. Door works perfectly now and did not have to buy a new lock.
While researching on the internet I found someone else had posted a similar method:
I have a 2004 Mazda 6 & this happened to me.
I have found the easiest way possible to get the door open without having to remove the interior trim.
1, wind down drivers window.
2, remove carefully the exterior window seal trim
3, about 6" down is the door handle lever, use a screwdriver to unclip the door handle & slide out the rod
4, use a wire coat hanger with a loop on the end or a thin piece of flat steel with a hole in the end
5, hook the wire into the rod & activate the door lock with the key & push the wire down & pull it all the way up & this should trick the lock. IT worked for me.
Was easy from there on & all that needed replacing was the spring inside the lock. A common problem I believe.
Either modify the spring or replace it.
Best of Luck
Arrowtown . New Zealand