I ran into this problem recently when moving a client’s WordPress site to a new server. On the old server, all of the pages had permalinks set up and they worked great. After moving the site, none of the pages were working, except the home page. All of the other pages gave me a 404 error. The admin pages worked fine.
When diagnosing this issue, you need to check a few things. First, check that your admin panel is still working when you log in. If not, you may have a problem with your base urls in the database. To check this, log into your phpmyadmin panel and check the wp_options table in the database. There are 2 entries that contain the site url. Make sure that they are correct.
Assuming that your admin panel is working, the issue is probably a server setting causing a problem with the Permalinks. To check this, go to the Settings>Permalinks page and change the setting to Default. Now, go back to your homepage and see if your pages are working. The permalinks will NOT work, but you should be able to click around using the navigation.
If your pages now work, then the problem is that mod_rewrite is not enabled correctly on the new server. If you want to check this, simply create a new htaccess file with the following code:
<IfModule mod_rewrite.c> RewriteEngine On RewriteBase /wp/ RewriteRule ^test\.html http://www.google.com [QSA,L] </IfModule>
*note that your RewriteBase will be different than mine.
Now, go to your WordPress site and try to navigate to test.html. It should redirect you to the Google. If not, then mod_rewrite is definitely not working and you need to have your server admin or hosting company fix it.
I found a lot of people with the same issue when I was trying to fix this, but no one seemed to have a solution. Hopefully, this will help.