The countdown made it all the way to T Minus Zero with "all systems go", and engine ignition actually commenced, but then, heartbreakingly, suddenly everything was not go; in fact, everything was kinda stop. Turns out the onboard computer monitoring software had noted a slightly high pressure reading in the combustion chamber of engine 5 (of 9), so automatic shutdown was duly initiated on all systems by the software and the launch was aborted.
It's a nuisance, not a crisis: Nothing blew up, and there is another launch window coming up on Tuesday, May 22, 3:44am ET (12:44am PT), by which time all performance data will have been examined, and the problem will be resolved. The solution may be as simple as tweaking the monitoring software a bit so it isn't quite so fussy about transient spikes in pressure readings!
From Scott: In other words, all things were not, alas, "five by five."