First, I'd recommend you get a new sensor and replace it before you drive the car any more.... either that, or put a mechanical pressure gauge on the engine and see what's going on. Its unlikely that your oil pressure would just die like that, but I'd sure want to make sure you have oil pressure before running. The alternative is pretty ugly.
Assuming its the sending unit (sensor), its easy to change... at least it was on my M119 (S500, 1996). Located right at the bottom of the oil filter housing; you can easily reach it from under the car. One wire to detach, then just unscrew it and replace.
When mine failed, it was the opposite of yours... oil pressure stayed at 3 bar all the time the engine was running.
The sender was about 60 bucks and I'm sure Phil right here at the Parts Shop could get you one pronto.