If you've installed an SD card, remove it and try the full power cycle. The SD card becoming corrupted on mine caused the same symptoms. Press the power button on the side until the pop-up asking about shutting down comes and goes and the unit is fully off. The timing on the power off pop up can be critical and the unit doesn't truly shut down if it's not pressed in time or the touch screen isn't perfectly responsive. Press the power button after waiting at least 30 seconds until the unit starts. If your problem isn't solved, return it immediately.
I don't have an SD card in there. It's done this since I took it out of the box two days ago.
I have never gotten the pop-up about powering off. This morning it just kept rebooting for 20 min, and I couldn't make it stop -- it wouldn't even turn off... just kept rebooting. I held down power, n, and volume up for 15 sec, and it flashed yellow and restored to factory defaults.
It went into the same power reboot repition again. This time, I was able to get it to power off (but still no pop-up confirming power off). And when I turned it on, it came on correctly. I'll see if it turns on okay tonight. If not, it's going to have to go back.