When the last time you used the SIM card? For your SIM and account to remain active, you need to have completed at least one of the following actions within the last 6 months:
Make at least one call, SMS or MMS to another number *
Make at least one connection to the Internet
Make at least one Airtime Credit or goodybag / gigabag purchase
* This does not including calls to Emergency Services or Member Services
If none of the above actions have been done, then your number will have been disconnected and all remaining credit will have been lost.
Should you want to continue to use the giffgaff services, you will need to obtain a new Giffgaff SIM card, activate the SIM and you will be allocated a new random mobile number.
You can order a new Giffgaff SIM here https://giffgaff.com/activate