Are you currently in nonpay status? Call 611 or # 1-855-478-2542 from another line to check your account status.
If you have been in nonpay/suspended status for over 90 days, or ported your Public Mobile number to another provider, then you lost your Public Mobile account, phone number, and access to self serve/My Account.
What or where are you getting the invalid SIM card message from?
Have you put the SIM card in another phone? Do you get the same message?
When was the last time you had active service? Once you call +1-855 4PUBLIC and enter your 10-digit phone number what message do you get? Does it give you your account status? What is your deactivation date?
Since you did a retail activation did you ever create a self-serve account? How long ago did this activation happen?
How have you been paying for service previously? Do you have a credit card on file? Or have you been paying by public mobile voucher?
Lots of questions but the more you can answer the better we can help you. Thanks.