Pretty obvious the OPs GFCI gave up the ghost and died when it tripped, it happens, things wear out.

But why'd it trip to start with?

A properly functioning tool, a good GFCI and a good extension cord can be a bad mix.

Understanding how a GFCI works can help.

If an electric motor is working properly, all electricity that the motor uses will flow from hot to neutral. A GFCI monitors the amount of current flowing from hot to neutral. If there is any imbalance, it trips the circuit. The GFCI senses a mismatch as small as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second.

A good but long extension cord can create a delay in the neutrals return time and cause a good GFCI to trip, it's just doing it's job.

It doesn't necessarily have to be a long extension cord, a long run in a homes wiring distribution can cause the same effect making one circuit in the home different from the others.

I recently had to remedy this problem at a friends home.
He has a small bar/kitchen with a frig on his dock. The entire circuit was GFCI protected as it should be on a dock.
The run to the dock was long, a 100 feet or so, he'd ran 10 gauge wire knowing there would be voltage drop in that long of a run.

The frig occasionally would fail when his GFCI would trip so he had recently replaced the small frig with a larger one, he often froze fish in it.

The problem got worse, it was intermittent, he didn't understand it.
His new frig smelled like rotten fish when I got there.

I took the frig off the GFCI and it was a fix.

But why was it intermittent, some times it worked and other times not?

Ambient temprature can play a roll, if it's hot weather a motor can pull more amps on a circuit when it is running, if it's really cold it can pull extra amps on startup. Ever notice fluorescent lights flickering when turned on when it's cold, it takes longer to ignite the gas in the tubes.

All these things and more will raise havoc with GFCI's.

Nothing necessarily has to be wrong with anything, it's just the nature of the beast...life isn't always fair.