I had this problem today for the first time after doing a normal power off. I tried all the suggestions for getting past the X, including holding down the power button for 30 seconds, multiple times. I finally found the procedure below and since my patience had run out I did the nuclear option. Back to square one - except it's restoring the apps I had installed! That's convenient!
1.If your tablet is on, power it off.
2.Press and hold Volume Up + Volume Down + Power button until the device powers on. You should see an image of an Android lying on its back.
3.Press Volume Down twice, until the screen says "Recovery mode."
4.Press Power to restart into "Recovery mode." You should see an image of an Android with a Red triangle after a few minutes.
5.While holding down Power, tap Volume Up
6.Use the volume down key to scroll to "wipe data/factory reset" and press Power to select it
7.Scroll down to "Yes - erase all user data" and press Power to select it.
8.After it says "Data Wipe Completed", scroll to reboot & press the power button to select it.