Possible reason may be the following (One or more than one):
1- Verify that battery is fully charged.
2- Check fuses and relays of Starter Circuit, Ignition, EIS, Fuel Pump.
3- Check Gear Selector / Brake Light Switch.
4- It may be Fuel Problems. Due to fuel filter Mercedes Won’t Start.
5- Check engine fault codes.
6- Check the Starter Motor. For the Problem of Starter Motor Mercedes won’t start.
7- Check the Ignition system, Spark Plugs/Coils.
8- Check Crankshaft position sensor. If the crankshaft position sensor fails you may notice the symptoms: Check engine light is on, Intermittent starting, Misfiring, Hesitation, Engine vibrations, Engine stall.
9- Check the Driver Authorization System. Key may not turn in ignition.
10- Other Possible may be: ECM / PCM computer failure, Camshaft position sensor issue, Shifter Selector module issue, Fault codes related to driver authorization or immobilizer, Electrical problems.
After checking all of those if the issue is not fixed please get in touch with the Eurobahn Mercedes-Benz repairing expert.