New (to you) car? Have you tried that key in the trunk, passenger door, or glovebox?

Maybe the key isn't for the doors. Replacing the ignition in W123's of a certain vintage is a time honored tradition. It usually occurs right after the ignition fails and leaves you stuck.

If this is the case and you have a key for ignition and none for the doors, AND your door locks are factory, you should be able to have the dealer cut you a new key based on VIN. I think it's about $10-20 for just a key.

Or, if you don't know the age of the ignition and you want a single-key option for opening and starting AND you know the door/trunk locks are original, then you can order a VIN-matched ignition and key set for around $100.