Just to chime in that I'm at the same frustrating place: no response to anything, having gone so far as unplugging the
battery and holding down power for a static reset.  In my case I witnessed the death of the Nexus 7 (2012).
The thing was fully charged, and I booted up from power down.  It was oddly sluggish (in retrospect, perhaps
it was occupying itself updating apps after a day offline) and I decided to reboot.  It came back up to the place
where 4 colored circles dance around each other, and never left that place.  After several minutes of colored
circles, I decided to power it down again, and it has been a nonresponsive paperweight ever since.