Is your website which was working without a flaw suddenly shows a 404 File
not found for all the pages. If this is the case then double check your error
logs in your server. If you are using a hosting provider then you can access the
logs from your control panel. When you read the raw error logs you would see
something like this. ".htaccess pcfg_openfile: unable to check htaccess file, ensure it is readable"
This means tat the .htaccess file was removed from your site. This could
happen if your hosting provider did some upgrade to the OS or Apache or some
other upgrade. They usually do this without notifying the website owners. When
they do that, all your site pages will throw an 404 file not found error.
To fix this, just check the permissions of your folder. For example if you
have a subdomain with a folder xyz. Then check the permissions of your folder
xyz. You can do so using the 'File manager' from your control panel. You can see
that the permissions have changed from 755 to 750. The last 0 means no access to
the internet user. Therefore you should change the permission to 755. That will
fix the problem.