The fluid would have gradually leaked until the reservoir was empty, every time you pushed the pedal, you would lose a little more fluid, until only air is left, It WOULD be OK one minute.. and gone the next, your description "dead cylinder" wether its the slave or the master, all that can go wrong is, contaminated fluid/air in fluid, cylinder bores damaged through corrosion, or the piston seal has deteriorated,, generally all three go hand in hand! very rarely its the pipe that connects the two together or something broken on the pedal!
Tom