I had a 406 that this happened to. It was the deadbolt on the door that had failed (normal mechanism OK)
I got a fixed price for the job, which was about £150ish IIRC, which was a right bargain as they had a right job doing it. The only way they could get the door open was to take all the interior trim off whilst the door was still shut, which meant taking the seat out. They then had to destroy the deadbolt with a cold chisel & hammer. they got the door open, but in the process punched a small hole in the body work (where the door shuts) so they had to fix that. All told it took them a day to do the door lock and had to have the car another day to fix the damage!