Nexus 7 won't boot up after the colorful X 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.

Good luck!