Depending on whether it’s the washer or the dryer, you should proceed differently.
Dryer: You can end the dryer program simply by turning the switch to “Ende” or opening the door. Then you can use the machine as usual.
Washing machine: Washing programs cannot be canceled. Please inform the AG about the problem. Additionally, you can request a refund of your credit by clicking the “Report Problem” button and following the instructions. You should be able to prove that you were unable to use the machine, so it’s best to attach a photo with a timestamp.