Just had a slightly anxious time with my Nexus 7. I'd inadvertently let the battery go flat, and when I turned it on this morning it briefly flashed up the usual start page then said "Connect Charger" and turned off.
I connected the charger, and got a blank screen with a briefly displayed "battery filling up" icon, but pressing the power button didn't do anything. I left it for several hours - still nothing. Pressing the power switch would display a battery icon and nothing else.
In the end, following a Google search, I pressed the power button in for about 10s and the Nexus rebooted, revealing that it was in fact 70% charged by then.
Perhaps everyone else knows that this happens when you get a flat battery on an Android tablet, but I didn't
.