I followed the official guide to download microPython and flash it onto an empty MicroSD card (SanDisk Extreme Pro 32GB), but the EV3 screen is now stuck at "Starting" after I insert the MicroSD card. The EV3 works fine without a MicroSD card.
This could be due to a bad SD card or due to bad flashing software. For example, there was a broken version of the Etcher software in early 2019 that caused the same symptoms .
I want to add that I encountered the same problem today with Etcher version 1.18.11. I tried to flash the image on my SD card 4 times with this version. Each time Etcher validation failed, and when I tried the SD card in my EV3 the screen stuck at "Starting".
Then I installed Etcher version 1.5.26 and flashed again: validation was successfull this time, and the EV3 booted correctly with the SD card.
So the solution of using version 1.5.26 (https://github.com/balena-io/etcher/releases/tag/v1.5.26) is still valid.