I'm with Mark. I suspect there's another GFCI outlet in some obscure location. Check the ceiling of the garage where the garage door opener plugs in or some similar obscure place.

Same EXACT thing happened in my garage and it took me forever to find the gfci outlet and reset. I assume you can get significantly cheaper wire tracers. I wonder if my network wire toner would work over that gauge wire...it a super cheap Chinese-made brand-less POS, but it's worked to trace network wires perfectly every time, even through walls.