Once your ESTA permit has expired, you will not be able to check it and the number will no
longer exist, therefore you will have to apply for a new ESTA authorisation. This means you
will have to fill in the form once again and you will have to repay the fee.
On the other hand, if you have just forgotten your application number, that’s not a problem
at all. You can retrieve your application by using some of the other information that you
wrote down on the form. When you are checking the status of an individual application
and cannot remember your application number, visit the government website and click on
“Check Individual Status”, located at the bottom of the page under “Check Existing Application”.
Once you click that, select the “I do not know the Application Number” option and there
you will just need to write your passport number, date of birth, family name, first name,
and country of citizenship.