Hi @hausi91,
Firstly, sorry for the inconvenience caused. We understand it is a difficult situation for our users but where we stand we can only look forward, we are doing our best now. As per our request, wordpress(dot)org plugin team has now helped us to auto-update some of the last versions to the latest version so more users are not affected by this.
The best approach to tackle this issue is: please contact your hosting service provider and ask them to restore to last working backup they have. These days most hosting service provider do have this backup service. Once you do this, please delete/deactivate the ThemeGrill Demo Importer plugin if you are not using it, if you need to use it, please make sure you are using the latest version 1.6.3.
Thanks.
Sanjip S.