This is most often caused by accidentally setting the customer account to Inactive from their Customer Account Information on the customer profile.
- Go to Orders > Customers and select the customer
- On the left menu click Customer Account Information
- Scroll down and look for the option called "Inactive" and make sure that it is set to No (if this has been switched to Yes accidentally, then the account can't be searched until it is re-activated)
If this doesn't resolve your issue, please submit a ticket for our support team to investigate for you.