Sudden brake failure

how is the fluid level and color in the brake reservoir? If the fluid is normal level and there are no visible leaks, I would flush the old fluid out and replace it until fluid at all the bleed ports runs clear. Start with the right rear brake bleed port (furthest distance from master cylinder), left rear, right front and lastly left front. Often times some dirt or gunk interferes with master cylinder piston sealing. Sometimes you get lucky and are able to flush out any schmutz otherwise the master cylinder would have to be replaced. If you have ABS, it is a flow through system and does not have to be energized/cycled while bleeding the brakes.