This is most often caused by accidentally setting the customer account to Inactive from their Customer Account Information on the customer profile.
To check this go to Orders > Customers > and select the customer.
Next on the left Menu click on 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.