Why is my customer is not able to log in to the customer portal?

A Contact is not allowed to log in to the Customer Portal beyond their support end date. You must update the Support End Date field in the contact record to a future date.

1. Log in to the CRM. 
2. Click the Main Menu.
3. Go to the Essentials tab.
4. Select Contacts. The Contacts List View opens, 
5. Open the Contact (who is not able to log in to the customer portal).
6. Click the Details tab.
7. Look for the Support End Date field.
8. Update the field to a future date.
9. Click Save.