Mercedes: Reverse Gear does not engage, No reverse, low fluid

Question
I have a 1990 Mercedes 4matic with Auto. Transmission. when I was parking my car in backward something blocked the tires so car was suddenly stopped even bounced forward a bit too. Since then my reverse gear does not engage. I checked the transmission oil it was very low. If I add the oil it might be Ok or there is some sort of safety cutting off. If there is a transmission safety, How can I reset it? Thank you so much for your help.

Answer
Majeed,
Did you physically observe the object you struck while backing?
If not, the sudden stop may be the result of something in the transmission seizing from lack of fluid. You should consult with a transmission shop that knows Mercedes transmission. They can tell you  about the correct type and quantity of fluid. It is very important to NOT operate the vehicle until the fluid has been brought back into the safe range, or serious damage can occur. I would suggest that you find a competent Mercedes mechanic to do a fluid AND filter change on the transmission. If the transmission has been hot because of the low fluid, there could be burned fluid in the case now and it should be vacated and a new filter installed before adding new fluid.
If the the fluid was low, it is a clear sign that the transmission is leaking somewhere and that need to be stopped too.

For the internal clutches and bands to operate properly and help keep the transmission cool, there must be enough fluid for the pumps to provide proper pressure on the actuators.
The fluid filter is probably the least expensive way to see if the transmission will operate properly.
Otherwise, a rebuild may be required to bring it back to operational conditions. Let me know when you discover the problem.
Good Luck
Rob