The power “switch” is a sliding lever that engages a microswitch (which does the actual switching). However the design of this is so %#*@ dumb as to make one despair for the state of the human intellect. If this happens again, get a small hammer, slide to the “on” position then give the body a sharp love tap right above the speed control dial. If it then comes to life you’re a victim of bad design. If you look at the picture of the dremel 4000 electrical assembly replacement picture earlier in the thread you’ll see a black box on the left. That’s a microswitch. The blue plastic “on/off” switch we humans operate is nothing more than a long strip of plastic that reaches all the way back to press this microswitch. The trigger on the microswitch is nicely rounded so that something sliding over (and travelling down the long length of the component) can easily operate the trigger. However, in the dremel the microswitch is mounted sideways so the blue plastic on/off lever (which is soft plastic) has to scrape up over the side of the trigger that has a sharp right angle. The trigger is also soft plastic. To make things extra schmart the microswitch isn’t actually screwed on to the housing, it’s only gripped by a ridge molded into the inside of the housing. So one of 3 things may be happening when you slide your power switch: 1. The plastic has been scraped off the blue lever by the sharp corner of the microswitch’s trigger, 2. The plastic has been scraped off the trigger itself, or 3, the microswitch isn’t being held securely so instead of the lever depressing the trigger, the whole microswitch part moves. All of these failure modes are covered by whacking the side of the tool just above the speed selector dial.