Prepaid Sim card issues

Good morning, 

  I purchased an O2 Sim card for 9.99€ from Frankfurt Airport and activated it. It had 3.5 GB data and unlimited calls. It worked fine for next day but later it stopped working. I'm unable to place a call as well as use mobile data. When I place a call it is asking me to recharge to continue. I don't understand the issue. The o2 app is in German and I'm not sure how to use it. Please help me out with this issue.

Thank you.