A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Dora,
It seems that you apply the template by using Settings>Apply a site template.
If yes, the existing site content should still be in your site. Based on the template layout, some additional content may be created and some content may be hidden. May I know what site content is missing? Could you go to Site Content to see whether all site content such as the site lists, the site libraries, the site apps and so on are in the site?
You can click on Settings>Site Information>View template history to see whether some content is hidden and some layout is changed.
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You can go to Site Pages (https://contoso.sharepoint.com/sites/sitename/SitePages) and check whether you can remove the additional pages created by the template and find the Home page and click on ...>Make Homepage to see whether you can get the original home page layout.
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Some factors may need to be changed manually such as the custom theme.
Thanks for your effort and time.
Sincerely
Cliff | Microsoft Community Moderator