If you let your Nexus 7 go completely dead, to the point where it has virtually no juice left, you may have issues charging. This bug has plagued me a few times in the past. Here's the fix:
To charge a dead Nexus 7:
1. Plug-in your dead Nexus 7 to a wall charger.
2. Immediately after plugging it in, press Volume Down and Power at the same time.
3. Once in the bootloader menu, use Volume Down until you see “Power Off Device.”
4. Choose that option with the Power button.
5. Once your device is off, unplug the charger and then plug it back into the device.
6. You should now see the battery meter (pictured above) with your device returning to life.
-From Derek Ross on Google+