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The SIM should be alive for at least 2 years if its a original o2 card. If it was deactivated you can request the remaining balance to be refunded (but for that you need a german bank account).

you have to write a informal letter stating that you want to cancel your prepaid contract and get the money refund to

Telefonica o2 Germany
o2 Prepaid Kundenbetreuung
90345 Nürnberg

glue the SIM on the letter, state the name and adress of the person who registered the card, state your bank acct data and sign the letter. refund takes about 2 weeks.