Applicants who have registered for an online account with USCIS in the past would have been assigned a "USCIS online account number" (not to be confused with a "USCIS Number," which refers to the Alien Registration Number, or simply "A-Number").
If an applicant does not know their USCIS online account number, they must visit the USCIS online account setup page, click "Forgot your password?" and follow the instructions.