PROBLEM: My 1997 C280 keys would not turn the ignition, wheels were straight, steering wheel was locked with free play. Suspected a bad ignition tumbler. In 10 minutes it was fixed, didnt cost me a penny.
SOLUTION: Inserted, wiggled the spare key, tapped it gently with a leather mallet, and managed to gently turn the key to position one. I then rotated the collar 90 degrees counter clockwise and removed the ignition tumbler using needle nose pliers (make note of ignition orientation to reinstall it the way it came out)
You will see a number of moving brass lock plates in the tumbler that move inward when the key is inserted (these plates cause the problem, due to normal wear and tear, and wont retract all the way inward as they should). Simply pull these plates out with needle nose pliers, being careful to stick them onto a piece of tape in the correct order, should you want to place them back again (but why?). Problem solved. Reinsert tumbler and collar.
Good news is the car starts and I won't ever have a sticky tumbler. Not so good news is there is a potential issue of being able to start the car with a slim flat tip, however I tried that and it wouldnt start... apparently it still needs the key. In any case this serves as a solid temporary fix IF you want a new ignition at a later date. Personally... I'm doing without a new tumbler, deciding to save $1000 bucks (what Mercedes quoted me for a new one installed by them).
Secondly, as happened with me, if the engine won't start after you reinstall the ignition, most likely the anti theft feature was activated. To deactivate, close the doors, then lock the doors with the remote, then unlock. That resets the anti theft... and the car will start.
Hope this helps.