My passenger door does this when it's cold.
Door lock actuator gets moisture in it (as a result of a breakin, not design related) and siezes up.
The linkages are pretty fragile as Triathelete stated, so venture into your door carefully when it finally lets you.
AW Golden lists replacement door 'guts' (that's everything except the handles) at @$150. I'm not sure of the availability or interchangeability data though.