Re-enable in Active Directory
1. Launch AD and search for the disabled user
2. Right Click their account and select enable
3. Hit OK
Re-enable in 365 Admin Portal
1. Navigate to Admin Center > Active Users > Search for user
2. Select Unblock Sign in
3. Uncheck the box ‘Block this user from signing in,’ and save changes
4. Confirm and allow for up to 15 minutes for the changes to update