Step 1: Sign into Office admin portal via http://portal.office.com
Step 2: From the Microsoft 365 admin center dashboard, Click on Admin.
Step 3: From the left pane, go to Security & Compliance Center.
Step 4: Navigate to Threat management > Review > Restricted users.
Step 5: Select the user that you want to unblock. In the Actions column, click Unblock.
Step 6: Click Next and then Yes to confirm the change.