Before starting this guide make sure the toaster oven is unplugged.

If the switch still works and isn't failing yet, simply try cleaning it.

Pry off the grey plastic on-off button to expose the head of the on/off switch.

Apply some electrical contact cleaner (CorrosionX or MG Chemicals Super Contact Cleaner or something similar) or, less ideally, 91% or better isopropyl alcohol to the switch.