Had you been controlling your speed with the brakes ( as in "riding them" ) just before they disappeared ?
If so, you may have boiled the water that can build up in the brake fluid. You should be changing the fluid every few years, to help keeping the water from building up.
When the water boils it turns to steam, the steam pushes the fluid back into the master cylinder. The next time you press down on the pedal, you have air ( steam ) in the lines. Air don't make hyd. brakes work.
When it cools down, the steam turns back to water, which will work to apply brakes.
Have your shop flush the fluid. That gets the water out.
You also need to control your speed by down shifting your transmission, if your not.
Remember, go down a hill in the same gear you go up it.
I had it happen to me, but in a much lesser extent, because I was in a parking lot, after the stopping at the bottom of a long hill.
Since changing the fluid, all has been well.