you will need to have it diagnosed by a good MB specialty shop. most likely, you have either a failed EIS (electronic ignition switch) and/or transponder key. they may suggest that you try another key; which you don't have. if it turns out to be the EIS, expect about $900 parts and labor. Even if you could order the parts, which you can't, they must be programmed to match your car with MB software or they will not work.
Could just be the key. Do you have a second one? How is th evehicle's charging system? A new EIS (electronic ignition module) could be more than $1,000.00. You also have pneumatic actualtors that unlock/lock the doors once the EIS recognizes your key fob.
When the "lock vehicle" button is pressed the remote control key sends a radio frequency and infrared signal to the corresponding receivers in the vehicle. These receive the signal and transfer the contents on to the electronic ignition switch (EIS) control module (N73) via the CAN bus. The EIS checks the code for access authorization. If the check indicates that the code is valid, the EIS transmits the "lock" command to the PSE control module (combined) (A37) via the CAN bus. The PSE control module then locks the doors pneumatically.
In addition the interior switches, i.e. the interior switch (CL) (S6s1) and remote trunk release switch (N71s15) are disabled.
When the remote control key is inserted into the electronic ignition switch (EIS) control module (N73) after locking, the interior switches are enabled again. The central locking is then in the same state as after locking with the interior switch (CL) (S6s1).