try making a call or sending a text, as standard giffgaff sims are deactivated after 6 months of no use , you can try asking a agent to have look into for you To contact an agent go to your My giffgaff page and scroll half way down on the left hand side you will find the "Ask an Agent" hyperlink. This is also the location to view the reply's sent by agents to any questions asked here is the link to contact an agent https://giffgaff.com/support/ask/
Please do not try a sim swapjust yet, i had a similar situation where the sim showed on my dashboard, and I tried a simswap and it messed the account up. It took a month to sort out via agents, so it would be better to contact them and see if they could suggest the best way to take it forward. This could save you a lot of hassle in the long term.