What do you do when the MI phone refuses to light up during a phone call?

This is because of the Xiaomi phone system settings, which prevent you from accidentally hanging up a call due to the proximity sensor. To fix this, you can do the following: 
Solution: 

1.  During the call, the face will block proximity sensors (proximity sensor in the phone on the left earpiece, a closer look will see two secretly dot oh), so a black screen is normal. If you move your face away from the screen, the screen will automatically light up, allowing you to record notes and perform hands-free operations.
2.  If you are using a sticker or some third-party film, the distance between the film and sensor block can cause this problem. 
3.  In the hardware detection interface (enter the hardware detection mode: In the dial-up interface, enter * # * # 64663 # * # *) follow the prompts from the sensor test, if the value of 0 or 5 does not fix the problem, you can choose calibration on the page. 
4.  Backup to restore to factory settings; 

If the above options are not possible, we recommend that you go to the nearby authorized service outlets for testing.