My credit balance Disappeared

I had a credit of almost 50 euros in my account. 
I purchased a 1.00 GB data volume package at 6.99.
All the credit in the account is gone / disappeared. 
I went to the O2 service at the O2 branch in Berlin. 
They did not understand what happened and to start surfing they charged me another 15. 
Now the balance is 5. 
How do you get all the credit back?

edit o2_Manga: verschoben von Prepaid zu Englisch Mobile