You can search using client's details such as email address or location to search for a contact by going to CRM > Contacts. If you cannot find a previous customer's name, they may have been deleted or set as inactive. 

You can also use the filters to further narrow down your search.

You can check the Show Non-Primary Contacts so that all contacts will show in your CRM.

You can also select the Show Inactive checkbox so that you can quickly see whether the contact has been archived or made inactive.