He message is ANTI THEFT MODE IS ENABLE. When it

How do I get my Mercedes out of anti-theft mode?

Look for a button that allows you to slide the physical key out of the fob. Step 2: Turn the vehicle key to unlock the car door without releasing it. Hold the key for 30 seconds in this position. This will signal to your car's system that you have the right key, and will enable you to bypass your car's alarm system.

https://www.google.com/search?q=MERCEDES+BENZ+C+CLASS+reset+security

..

Sounds like a security system problem. Do this, turn the ignition switch to start but don't start the engine, then leave the switch in the run position, watch the theft light it will be on steady or flashing, wait for the light to change then turn the switch off then repeat up to 2 more times, it takes about 10 minutes per cycle don't try to hurry and if the car accidentally starts and quits start over. Occasionally the vehicle will start and run after the second try but don't count on it, I hope this helps, take care.

It's hard to get the SECURITY light to go off when you don't know how it works ! Replacing the PCM could cause you more problems too . If a replacement PCM is being installed it must be programmed (flashed) using the latest Techline programming procedures.
Having the vehicle check for DTC ' S - diagnostic trouble codes would be the first step.
DTC P1626 Theft Deterrent Fuel Enable Signal Lost
DTC P1630 Theft Deterrent Learn Mode Active
DTC P1631 Theft Deterrent Start Enable Signal Not Correct
DTC P1632 Theft Deterrent Fuel Disable Signal Received

This anti-theft system does not use a TDM - theft deterrent module . The instrument cluster is the controlling module . Look at a wiring diagram here
http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model engine . Then under systems click on body & accessories , then in subsystem click on security / anti-theft . Click the search button , then on the blue link .You mite be able to do a by-pass if the contacts inside the key lock cylinder is the problem . You can check it with ohm meter . The key has a resistor pellet on the side , you can check the value with ohm meter , find the two wires under the dash at the steering column . Two white wires inside a yellow or orange sheathing . There are videos on youtube !

To turn off anti-theft mode:

1) Turn off the monitor
2) Press and hold the menu button
3) Turn on the monitor

Release all the buttons
that should disable the anti theft mode

you could pull the fuse disabling th anti theft totally.. not sure that would work though

anti theft mode enabled my monitor pls tell me about

first power off u r monitor
then press and hold menu button on the monitor then press power button on monitor

my acer monitor showing anti theft mode enABLE

Probably wasn't the starter anyway. When this message comes on you will need to replace the ignition lock cylinder.

you should be able to use the key to unlock the driver door or the back hatch and that should disable the anti theft.