Try working the steering wheel back and forth while attempting to turn the key. If you have the wheel turned, the pressure on the lock bolt can make it impossible to turn the key. Releasing the pressure will free the bolt.
If it frees up, but the key is still "sticky" you need a new key and/or tumbler.

Is that a real steel MB key or a brass replacment, and how old is it? If old, or brass, get a new key (take your registration to an MB dealer or good independent, and they will order you a new key). A new key will often work in a tumbler a worn key won't turn.

If the new key works, you also need a new tumbler (about $60, key is about $20 last I knew) -- fairly easy to replace IF the key turns. A huge PITA if it doesn't....

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!