Sorry to hear of the problem you are having.
Please check whether the solutions posted in the following topics help you to resolve the issue.
If you can’t resolve the issue then please tell me which demo content you want to install on your site and would you mind if i log in to your site and do some troubleshooting ti import it? If this is ok then could you please share me your site log in details and cPanel or FTP details privately by adding them in the box having text “Click here to share private content. Only you and forum moderators will bee able see it.”?