Electrical – GFCI Tripped – Cannot Reset

Not an uncommon problem. I have had a lot of problems with GFI's the last few years myself. There are a few likely causes:

1. A fault still exists and will not allow the GFI to reset.
2. The GFI tripped due to an overload and the differential circuit was damaged, thus the outlet is now toast. This seems to be a common complaint with standard 15 amp GFIC's. they don't take current spikes well. In your case 1500 watts on 120VAC is pulling around 12.5 amps, which is getting close to max capacity. Sometimes the initial current draw is much higher as the resistive load needs to heat up a bit before it reaches its operating impedance. A "cold" load can offer a much lower impedence and cause a short duration, but large spike of current to flow at turn on. Very common on motor loads, but not uncommon with filament heating or lighting.
3. Sometimes if you turn off the power to this GFI at the breaker, then push the test and reset buttons, then turn the power back on again, the GFI will default to the tripped condition, but will reset.
4. Disconnect any wires connected to the "Load" side of the GFI. Unplug all loads from the GFI itself, and give it a try.
5. If none of the above items work for you, you will probably need to replace the GFI. Find one with a 20 amp rating instead of 15 amps, assuming that 15 amp is what you have now.

It's possible that your GFCI outlets are mis-wired in a daisy chain. What happened then is that when the cord cutting event occurred all the GFCI's on the circuit tripped. While you found two there is likely another.

GFCI's must have a power feed to bevreset. If they are improperly daisy chained they must be reset "in order", as the first one provides the power feed to the next. They can not be reset out of order.

If this is the case you can choose to live with the situation as it is and just "know" the secret to resetting them. Another choice is re-wire the circuit so that the GFCI's are no longer daisy chained and each can be reset on its own accord. If that route is taken the circuit should be carefully evaluated so that GFCI protection is not eliminated at a location requiring GFCI protection (such as that standard outlet in the garage that you mentioned in the question).