There's a plastic "lever" going from the key mechanism down to the door lock mechanism. Most likely it has broken off. My SDL had this issue when I first got it due to someone forcing it with a sticky lock mechanism. The key should be very easy to turn in the slot, if it feels stiff, the mechanism needs lubrication and can/will break the plastic lever.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]