Take it apart as you would to replace the battery (see instructions here or one of the many Youtube videos. The 'on' switch is a snap-type of switch, whose contact are likely corroded (they are copper). Take the chrome retainer/cover off of the top of the switch by prying one of the two sides that are clipped over the switch (a magnifying headset or glasses would be helpful). The chrome 'snap' button just lifts up, revealing the copper contacts. Scrape/clean these, as well as the underside of the round 'snap', and clean with Deoxit afterwards. Test the switch contacts by shorting them with a small screwdriver to turn the toothbrush on and off. If it doesn't turn on, it is not the switch, but something else - perhaps the motor (brushes wear out). If the motor turns on and off, then re-assemble in the same order. Use tweezers to pry the top chrome cover back over the switch and bend back under to secure. Test switch before putting the toothbrush back together.