Hi - it sounds like the transmission has gone into limp mode - this happens when the transmission detects certain faults and prevents the box from shifting gears.
In the first instance the vehicle will need to be checked on a diagnostic tool such as Mercedes STAR or similar to see exactly what codes are stored.
A common problem can be with the speed sensors on the valve body - on this model the complete valve body will need to be replaced as it comes as one part from Mercedes.
This will of course need to be confirmed with a diagnostic check before any repairs are carried out
Best regards, Paul