My Nexus 7 will not boot up past the start-up animation (colorful X)-it's not a power problem as it charges up OK-just will not get past the X. I've powered it up and down several times but it hasn't solved the problem.
I had a similar issue - only seemed to happen when power had got really low and wifi was on. the way I got around it was:
Connect to charger Shut down nexus Hold "volume down" and power button until it wakes up On the bootloader screen, press up until it says "recovery mode" Press power You should see the Android logo with a red exclamation mark Press and hold power, and then press "volume up" which should take you to the recovery menu. Once in recovery mode, leave plugged in and fully charge (several hours). Once fully charged shut down and restart and it should get past the X.