Assuming that the SIM is active, if you insert it into a handset and text NUMBER to 2020 (text is free), it will report what the number is.
Additionally, you could text the word FORGOTTEN to 43430, and you’ll be sent a confirmation of your correct username (usually within 15 minutes) as well as the e-mail address that was used to sign up.
You can then have your password reset by going to: http://giffgaff.com/support/lost-password and entering either your username, phone number or e-mail address, allowing you to access the account and use it as normal.
If, however, the SIM is inactive, it might have been deactivated due to lack of use; do you know how long it’s actually been since that SIM was last used? In order to keep a giffgaff SIM connected to the network, it is necessary to do at least one of the following every six months:
- make a call, or send one SMS or MMS
- connect to the internet
- make a topup/purchase (credit or GoodyBag)
In all cases, usage whilst outside the UK would count towards this requirement.
Unfortunately, if your SIM hasn’t been used for longer than that, it will have been permanently deactivated, and this cannot be undone.
There are more details in the KnowledgeBase article at https://www.giffgaff.com/help/articles/why-is-my-phone-number-deactivated and you can also review the official T&Cs at https://giffgaff.com/terms#Cancellation