Brand new Mi A2 (Global Version), from the shop today and went through System updates - through, to Google January 2019 Security Update and the device will not boot; same problem as with everybody else here. Argh.
Try to start it with holding both volume buttons down and power - then release after logo and, also -perhaps- volume down.
EDIT: Ah, well, thank God (LOL). It looks like there isn't anything wrong with Google's "Security patch level: January 5, 2019" (Android 9)!
However, because this happened through normal operation - it seems that a LOT of people *are* going to have this problem...
How many know that you should Factory (Data) Reset after a (regular, OTA) update; and how many will be able to boot their device, out of the loop. Geez!
P.S. Btw., you owe me *beers! ;$
EDIT: Zomg., this thread is a duplicate, already @