Hello running girl, Have you looked to see if maybe the "lock-off" feature hasn't been engaged?
If you look at the razor, right above the on/off and sideburn trimmer selector, you should see a full bar that goes all the way across the shaver. Push this up toward the shaving head to unlock the razor and the shaver should work fine again.
This is a Braun design feature there to help prevent the shaver from accidently getting turned on when packed in your luggage, etc. and running down a fully charged shaver.
There is nothing like reaching for what you thought was a charged and ready to use shaver and discovering that it got switched on and ran the batteries down.
Other than having the lock off feature engaged, I can't think of anything that would cause the switch not to turn on, unless the shaver was dropped and damaged. If that's the case, you will want to have it looked at to make sure the printed circuit board hasn't been broken, etc.
Typically this isn't something that most folks can undertake a diagnois and repair, so I'd suggest you contact www.electricshaver.com
or call them at 1-888-476-1785 to get an estimate for repairing it.
Hope you find this Very Helpful and best regards!