Unplug it from the keyboard and never shut it down while plugged into the keyboard again! This has been a known issue since these things came out.
I thought I bricked mine when it happened to me! Just always remember to take it off the keyboard when you shut it down until the LED begins fast blinking again while plugged into the charger and then you can reconnect it to the keyboard. Just remember to always remove it before powering down.
To get it restarted now, get it off the keyboard and charge it for a while -hours to overnight until you get a solid power LED, then try the 20 second long press on the power switch, with it off the keyboard base, and then immediately try 3 presses at about 1/2 second interval and see what happens. Try it plugged into power and unplugged both.
As far as I know, this is a situation that has been around with these units when connected to the physical keyboard dock since the very beginning. I think I had it happen twice way back when I bought it around 2015 and didn’t know it was an issue, and then forgot another time. What seems to happen is that it looks like it shut down, but somehow powers back up and runs the battery down to nothing while connected to the dock. As I recall I had to plug mine in and let it charge a couple hours until I hit on the correct power button reset combo. The 20 second long press apparently guarantees it’s fully shut off. I guess the three clicks get it going again,. No idea why it takes three rather than just a single press, unless that is an internal reset for the electronic switch/battery management chip used in these devices. All I can think of, really.
Let us know if this works for you, but get it off the dock to get it turned back on and don’t EVER shut it off while connected to the keyboard dock in future. You can reconnect it to the dock once the LED has started fast flashing to Indicate it has shut down completely and is now fast charging if you leave it plugged in. Otherwise, give it at least thirty seconds after the screen goes off before reconnecting to the keyboard/dock and folding it closed. I think it only happens if you shut it down while connected, but can’t recall if it was an issue if you just closed the tablet on the keyboard to get it to go into sleep mode. Once you figure out how to reset it again from what I’ve written you can try it if you like, I just avoid it completely!
BEST OF LUCK! Please let us know if this gets it going for you. It has worked both times it happened to me.