The thermal reset button pops when the motor gets too hot...usually because a bearing is going bad. Don't wait until 2 am on a January night to replace the motor. Take things apart, make sure the gearbox input shaft spins smoothly, make sure the fan coupler is in good shape, and clean the fans and housing. It will take some patience to get that fan off the motor shaft, but penetrating oil and patience works wonders. If you can have a friend stop by and give you a hand, one person can put a little pressure on the back of the fan hub with a screwdriver or whatever, and the other gently taps the motor shaft.
If you aren't comfortable doing this work, it is time to find some help.