Last time i pressed the brake, felt a pop, and lost pedal i had a pad explode. It let the pistons overextend into the rotor and tore the seals.
It could be the anything from the seals in the caliper to the lines, to the master. First check to see how much fluid you lost, then start looking for the leak. Most brake failures happen in soft spots like seals and fittings and so on. Typically i start at the corners and work my way in and up to the master cylinder.