Phone is Black After Logging In
Sounds like a software bug.

No real solution here, but a plan of attack toward discovery of the culprit(s).

Try to pull the battery, and restart. If the battery is non-yankable, it'll have to discharge fully to kill all processes. I usually expedite this by leaving it within reach of my neighbor's WiFi, where it exhausts itself trying to Connect...Connect...Connect.

Charge some, and restart, until the buggy situation becomes clear to you through experimentation, again. I say charge SOME, because if you charge fully, it'll require double patience to discharge again.

Let us know what you find out.
For myself - I believe this occurred when the Amazon Kindle app was trying to download books that I sent to the device via computer. It MAY HAVE STILL BEEN DOING SO WHEN I LEFT THE WIFI AREA (pure supposition). The first time did not occur to me, but the second time it did it (for a trip), and I recalled it happenned roughly similar to the first occurence. From now on, I'm careful to gauge full downloads while the device is awake, and Close the app fully. Hasn't recurred in a few months.