Your file isn't XML-compliant

Has anyone encountered this error message before, or knows what it means and how to correct it?

What happened:

• We acquired a company, and I accidentally overwrote our root site. (I accidentally migrated acquired company's root site over our company's root site.)
• We did a carbonite restore of the root site, which reverted everything back.
• And now when I go to the home page (companyname.sharepoint.com) it comes up with this error message (see above screenshot).
• I can get to the Search page (https://companyname.sharepoint.com/search) and conduct searches.
• I'm able to get into the Site Manager screen (sitemanager.aspx) and pull up the Site Master Gallery and preview the Seattle master page.

I inherited this SharePoint site from the previous person in-charge, which is no longer with the company so I don't have any background knowledge of how they created it. However, I spoke to the IT Manager, and he did mention the previous person made some custom changes to the SharePoint template, whatever that means. We have no idea what was made.