The requested page does not exist.
WHAT IT MEANS
The server could not find that address.
IF IT IS ONE PAGE
The page was deleted, renamed, or the link is wrong.
Check the URL character by character. Case matters on our servers: About.html and about.html are different files.
IF IT IS EVERY PAGE EXCEPT THE HOMEPAGE
Rewrite rules are missing. This is the classic post-migration symptom.
In WordPress: Settings > Permalinks > Save. That regenerates the rules.
For other applications, check that .htaccess transferred and contains the routing rules.
IF IT IS EVERYTHING INCLUDING THE HOMEPAGE
The document root may be wrong. Check cPanel > Domains > Document Root.
Or the files are in a subdirectory inside public_html rather than directly in it.
AFTER CHANGING URLS
Every old address needs a redirect to its new location, or every link and search result pointing at it breaks.
MONITORING THEM
Search Console reports 404s. Analytics shows which pages people reached them from.
A pattern of 404s on paths you never had is usually a scanner probing for vulnerabilities.
