The most obvious thing to check 1st would be the battery. Call AAA or whatever roadside assistance plan you have and get a jump start, maybe?
You have two batteries and this is somewhat unusual for your Mercedes. But your situation is most often related to dead batteries; not enough juice to light the lights or to turn the starter motor. Jump start the car properly; leave donor hooked up long enough to charge your battery, at least ten minutes. If your car starts, fine. If it does not start, replace your battery. After either of these, go to an auto parts store, or better a Mercedes mechanic, and have any codes read. They may point you to the reason your battery died. Most likely it's just a 2015 battery that has deceased, or it might be a defective alternator that hasn't been able to keep up with the batteries.
Search Mercedes-Benz C-Class Questions
Mercedes-Benz C-Class Experts