Please Help!!! 1996 Mercedes s500 stuck on limp mode. Won't shift?

Hi there I have a 1996 S500 that's stuck on limp mode and there's no check engine light. It happened right after I put in some regular gas, which I started regretting while in the middle of fueling. Anyways after I drove it for 20 miles it went into limp mode its stuck on 1 or 2nd gear I believe. The rpm goes over 3000. Its really heavy when driving it. And I tried taking out the battery, erasing all the codes and also turning the key on and off 5 times but still. So now I don't really know what to do or what the problem could be. This happened a year ago but I got it taken care of after I had it reprogrammed but this time after I did that it didn't work. Thank you all comments would be real helpful.