Interestingly it's happened at least twice with my new iPad Air. The old one (Series 2) didn't crash ever, well not that I can recall. Guess that proves to show that with progress comes higher degree of system complexity, and new variables. What happens or happened? Well the thing just freezes, no response, a black screen is all that glares back at you. Fortunately it's in most cases a quick fix, here's how.

Rest by pressing home & sleep/power

Which is your only option and it works.

- press the home button and keep it down
- at the same time press the power button
- hold them for some seconds (>5 I seem to recall)

your iPad will reboot, when you see the Apple logo on screen you can release the buttons.

Why does it happen?

Could be many reasons but I noticed the last time it happened to me Safari had closed/crashed on some certain web pages, repeatedly. Without knowing for certain it therefore could look as if the locking was a protection instance, or a routine sequence that demanded a hardboot or reset after crashes.

Going back to the same web page that caused the crash, after the resert, everything works and there's no crashes, or repeated symptoms. Ah yes, the web page in question had slides on it and quite a bit of scripts. Pretty certain that could be part of the reason.