Since reverse is the only gear you're missing, I would bet that the reverse band adjusting bolt has backed out(very common). If not that, then the reverse band is broken! The band adjusting bolt is right above the Kick-down solenoid on the right side of transmission. Have a reliable technician perform this adjustment.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH