I have an SL500 , 2004. One day it just went into first gear and reverse only and the engine light is on. It does not shift into second third etc. No warning. I was told the trans goes into limp mode. Has anyone had this problem, and what do I do/ or ask the mechanic to do, and how expensive.

 

MB makes a very complex vehicle, yes many have had LHM for several different reasons. The transmission shifter is not a direct connection to the gears, like years ago, so electrical and vacuum factors can be part of the issue.
If your vehicle is a 2004, that is wonderful but this forum is for us older SL's You may get better advice from the newer (R 230 ) owners!
Where in the world are you? Welcome to the forum and good luck with getting into gear. I always feel a dealer opinion is a good start.