I've had this exact thing happen three times with my 1617EVS. Turns out that (as mentioned by others) the switch itself gets packed with sawdust so that the contact will not work. With patience you fix it as follows:
Safety: Be sure you have the machine UNPLUGGED before you work on it.
1) disassemble the top of the router - two screws if memory serves
2) unplug the spade lug connections from the switch (remember where you parked so you can find your way back - take pictures if necessary)
3) with a small screwdriver, pry the plastic sides of the switch to let the back release. Be careful there are small parts that will fall out like the contact bar and spring. Take it apart slowly so that you can put it back together, sans sawdust, the same way it came apart.
4) Clean out all the sawdust and clean the contact parts as necessary
5) put the switch back together, test it first with a meter (continuity)
6) reattach the spade lug connections
7) make sure all the wires are back in the right place so they don't get pinched when you reassemble to top.
8) reassemble the top
It should be ready to go. As I said, I've done this now three times and so far never had to replace the switch. But the inside of it is always packed completely full of sawdust. Thinking about it now, maybe sealing the switch back with a light film of silicon caulk or something would solve the problem. Might be worth a try. If you keep air from flowing through the switch then it should not accumulate dust in there.
YMMV - could actually be a bad switch. I have no affiliation with Bosch.
Paul Proffitt