To fix a "Site Currently Unavailable" error, you will need to edit the page. The only way to do that is through the content library.
Navigate to the Content Library and type in the name of the page, which can be found within the URL.
After you search for the page in the Content Library, hover over the Title link. This helps confirm it's the right page. Then in the bottom left of the screen you should see the matching URL.
You can check the page to find the broken file link. Usually it's Microcontent, like a Link Collection List, that points to that file. We will choose the Microcontent and click the gear icon. Then, we will check if the broken file link is in it.
Click the link in the collection. This will show you a dropdown window. You can find out which link leads to a deleted file there. If says "No results" then you know this is the broken link and you can remove it by selecting the remove button.
You can tell a link isn't working when "Media" shows up in the dropdown along with the media item info.
Once you save the fixed link collection, the page will be fixed, and that error message will go away. This is why it is important to only permanently remove files that are not being used on the site.