As a recent user of Freshdesk, you would want to bring in all your contacts into the portal and while doing that - you get an error on the customers' tab saying "an import is already running." This is because while uploading the CSV (or excel) file, there would have been a technical glitch which would block the import. Kindly contact support (support@freshdesk.com) and ask them to kill this so that you could import the file again.
Another recommendation would be to check the solution article in the "file import" page which would give you specifications about the various parameters in the file.