Car key won't unlock door?

linkage connecting lock cylinder is disconnected  from the actual  locking mechanism.  

There is a clip that hols the linkage to the lock cylinder. It is usually plastic but on some cars, metal.

You have to open the door from the inside, remove the door panel, use a strong light (cell light and camera is a good idea), inspect what is broken or popped off.

Then see if you can buy the part from your local parts store.

There is a 'U' shaped clip that holds the lock in place. Remove that and the lock pops outside the door for easy clip replacement.

Replace clip and connect linkage (or cable as applicable), reinsert the lock, replace the 'U' shaped clip, and test before replacing the panel & closing the door.

You may need to be a contortionist to reach the parts as the space is usually quite cramped.

It is a good idea to lubricate moving parts while it is open.

And spray silicone lubricant on the felt window channels.

Remove the door panel and start inspecting.  Everything else is just a guess.  Could be the key barrel, could be the latch, could be the locking mechanism. 

And the "no stupid answers" comment is not NOT needed.  You have not parted with any of your hard earned cash for FREE answers.

https://www.youtube.com/watch?v=8OGjMrjvQc0

The mechanism inside the door has disconnected, check for a YouTube video telling how to fix it.

Either the rod from the key lock to the door latch mechanism came off or the latch mechanism is worn out and broken. Take the door intrior panel off and see if the lock cylinder rod is still attached.

It done be broke . . . if you can't fix it you can either have someone else fix it or leave it that way.  Very simple.  There is no pill, potion or prayer that will fix it.