The best answer to this IMO is grneyerdhd's charging via computer.
After a minute or two it magically came on when I pressed power for a few seconds.
Shame on ZTE for overlooking this!
I am hoping this answer will be voted up so he gets all the credit. Terrific discovery while the rest of us thought using the factory charger is best.
When this happened with my LG G3 I had to reinstall battery. Shame on android too for not putting safeguards for this where they let the entire battery get drained.