Refund of Prepaid Credit After Switching to Postpaid Plan

Hello,

I recently switched from a prepaid O2 plan to a postpaid plan under my husband's O2 contract, keeping the same number but using a different SIM card. This change was made at an O2 shop. However, I still had prepaid credit and intended to submit the Prepaid – Guthabenauszahlung form to get the credit refunded. Since my prepaid account was canceled, I can no longer log in with my prepaid credentials to check my remaining credit or see what happened to it. Could you advise me on how to proceed in this situation?