How to reset the display of nodes?
Wanted to post my solution(i encounter it too) if anyone stumble upon.

You don't have to uninstall and reinstall fieldgroup if you know which content type is causing this issue. 


Go to Configuration Export ( [your URL]/admin/config/development/configuration/single/export)
Choose Entity View Display in  Configuration type
Look for your content type in Configuration Name
Copy the configuration that appeared in Here is your configuration:
Open a text file and paste your config.
Look for field_group: , delete it together with it's child or until id
Then re-import your config. 


That works for me without resetting other content type field groups.