This is strange: neither door lock nor the tailgate lock on my '87 W124 TDT will turn using the key. What would cause all three locks to become immobilized?

For the record, the central locking system works from the inside of the car (i.e., pushing down on the lock button will lock all doors).

Thoughts? Hoping there's a simple answer.

Many thanks,

Corvo Gold
87 W124 300TDT
88 W124 300CE
65 Corvair Corsa
65 Corvair Monza