More likely failure to pressurize rather than depressurization. Either should have set off an alarm, so neither one is likely without multiple systems failures. But failure to pressurize (along with a defective alarm) is more likely to take the pilot unawares than a rapid depressurization event, alarm or no, because the effects of a rapid depressurization at altitude (ears popping, formation of fog in the a/c, etc) tend to be obvious.
And the first step in the Quick Reaction Handbook (which is the part that the pilot has to commit to memory) in the event of either depressurization (or smoke in the cockpit) is to don the oxygen mask. So if the pilot had even an inkling of a hint of a depressurization, he should have had on his oxygen mask in single-digit seconds.
It's more likely he went to sleep in the climb because the a/c wasn't pressurizing.
Or maybe the pressurization was fine and the pilot the stroked out and none of his passengers noticed anything was wrong until it was too late.