You need to unlock the bootloader again. Keep your phone in fastboot mode (that's needed anyway for unlocking), and use the Xiaomi unlocking program on your PC. Log into your MI account, then attach your phone (while in fastboot mode), and proceed for unlocking. It should work, otherwise please come back here with a screenshot of the PC to show the possible error you get.
If you unlocked the bootloader in the past, there is no need to boot into Android to do anything. The important thing is that your device is bound to your Mi account, which will be the case anyway from your first unlocking.