Will not start/key will not turn

First ensure the simple thing like the battery, will your headlights come on bright.
If yes turn the key 180 degrees and reinsert it and see if this works. If you look at the end of the key there is a red dot. This “window” is used to send information to the EIS (electronic ignition switch). Next would be the spare key, try that. If you reviewed the owner’s manual you will note that this car came with three keys. So make sure to try both additional keys. Reading your message I assume the “shunk” sound you normally hear is not there. This is the steering lock moving. Lastly try taking the key in and out of the EIS this will take time and patience. If this works you will need a new EIS.

Hope this helps

Benz key pr-programmer is 200 euro, on Bynkaay company.