To allow a user to log back in again, you will need to log into the Microsoft Admin Center as a Global Administrator.

Go to Users > Active users in the side menu

On the right side of the page, search for the user who was locked out on the Control Panel

Click on the corresponding user in the list

Click on “Unblock sign-in”

Untick the “Block the user from signing in” checkbox and save the changes

Microsoft gives you a confirmation the user is now unblocked from signing in. However, it may take up to 15 minutes for them to be able to sign in again.