We have a Sennheiser ew 100 G2
TOTALLY love the mic!
Here's our problem;
-
The small powerbutton in the "butt" end of the mic, has slipped down inside the mic handle.
That switch is likely mounted on a printed circuit board. Either the switch has been torn off the board, or the circuit board itself has been damaged. Your best bet is to have it repaired by Sennheiser.
I've repaired one of these before. The best option, as FMEng mentioned, is always sending it in. That being said, if you are on a time crunch, it's fairly easy to open it up and realign the button. We've shoved a small chunk of foam on the other side of the PCB that the button is mounted on. It tends to hold it in place a bit better.
Unfortunately Sennheiser no longer repairs the G2 and earlier stuff I believe. (at least in terms of replacing damaged parts) If they were able to fix it with just the parts with in the mix it self they'd do exactly what you are able to do, and as MrMaglit and FMEng have mentioned. I opened up 2 of these recently that got some water damage to them. They aren't obvious as far as opening up. Unscrew everything you can, then remove the 3 Hex screws around the base of hte batter compartment, next to the transmitter portion. The Circuit Board is actually attached the Battery Slide and the outer casing slides off the end.