key won't turn
your cumblers in the hole for your key may actually be stuck or jammed in by dust,dirt, or metal shavings from another key. You can fix this by either using a can of dust off to try and spray the debris out. However if it still does not work you may need to replace your ignition column.
however if this isnt fixing anything you may have a electirical problem. but ive had to deal with problem a couple of times and the way i explained earlier has worked for me most times. you could also use a paperclip to help clean out the key port.
Sometimes as well the transmission shifter or steering may be binding the key.... Try and wiggle the steering wheel and shifter while trying to turn the key.
I am assuming you meant ignition key not door key
Looking for a Used C-Class in your area?
CarGurus has 32,183 nationwide C-Class listings starting at $1,200.
Search Mercedes-Benz C-Class Questions
Mercedes-Benz C-Class Experts