Key wont turn on a 2001 s500 benz ws working yesterday gear shift also will go out of gear without the key

SOURCE: replacing the battery in a 2001 mercedes benz

Battery base has a device that clamps the battery on a step/notch on the side. Use a ratchet with an extension to loosen. Once it is loose you can remove the battery.

SOURCE: My 2006 Chevy cobalt key will not turn completely

ur gear shift has a little plastic bar on gear shift and it has crack on the side u cant see have it changed out and both of ur problems will be solved at same time.

SOURCE: Merces Benz s500, Ignition key does not turn, can

Check Battery in car(make sure is not weak or dying), check battery in remote fob, possiable draw in system possiable fuse or possiable keyless entry module failure

SOURCE: I tried to put my Ford Escape 2001 in gear

sounds like the link on the box has poped off have a look under the car at the selector arm if its not moving the shaft its the gear linkage in the car

Testimonial: "Thanks, MikeT! The cable had popped off at the transmission. My handy beau put it back on and secured it with a metal retaining cap from the h.store."

SOURCE: My 2000 Mercedes Benz S500 is dead- electronic key does not

It's not that unusual for a battery to fail that soon, and the "no lights, no nothing" says that it's the battery again. Your best bet at this point is to pull the battery and take it to the point of purchase for testing. They should either replace it under warranty or charge it for you. After you replace the battery you should have the charging/electrical system checked for possible shorts or power drains.
BillB