Hello Franziska Jenetzky,
* Disconnect the battery and reinsert it as you did. after that Press and hold together Volume Down + Volume Up + Power button for a few seconds to enter the recovery mode.
****use volume keys to navigate and power key to select/Confirm
select option on screen # "wipe data/factory reset" and press power on button to confirm
once complete the process, select reboot option and use power button again to confirm
* phone will be restart
Hope this answer will help you to fix your phone.