Hi, what date approximately did you activate your previous sim
(when did you join giffgaff)
Once you bar your sim, you can't then log in with your number, you must use your username
Is the username similar to the one you're logged into?
Could you attempt a guess at it?
What date did you last top up or buy a goodybag?
And did you receive an email from giffgaff confirming that transaction?
Your correct username will be mentioned within that email