Is there a red security light flashing on dash ?
if you have the radio code because we going to needed after you connect the battery back up to make radio work ,we can try a master reset if you have not tried yet
if the reset dont work ,you will need a diagnostic scan with a star Mercedes diagnostic scanner or equivalent to retrieve fault codes.
it could be number of reasons, if the key circuit board went bad it will cause it, the ignition switch itself etc...
Master reset Procedure
1- Disconnect Battery Cables ( Negative & Positive Cables )
2- Hold cables Together for 60 secs ,then let go
3- Wait 15 minutes
4- Make sure all doors are closed
5- Reconnect battery Cables
6- Lock & unlock the vehicles using the key from driver door side ( the metal blade key , in the key fob)
7- Enter the vehicle and attempt to start
Please let me know
ASE Certified Technician