Hi, are you sure you are logged into the right account because it should let you. Check to see if you can see your number on the dashboard and some recent transactions in your purchase history. If you can't then you are in the wrong account. If you can, then clear your cache u and cookies and try again.
To retrieve your password, request a reset link by putting your mobile number in the lost password link belowhttps://giffgaff.com/support/lost-password
If you don't receive an email with a reset link, (check in your junk folder as well as giffgaff emails do end up in there)
Text the word PASS to 43430
you'll receive confirmation of your correct username and the email address you have in your giffgaff account details (could take up to 5 minutes to arrive)