Either you have just installed WordPress or have moved your website to a new server/domain and you are now only able to see the homepage of you site. When clicking on a link on the nav you get a 404 error.
Don’t worry the fix will be simple. From previous experience it will be one of the following:
1. .htaccess – this hasn’t copied over to your new directory. Check for the .htaccess file just inside your parent directory and check its contents. If missing recover from your previous directory.
2. Permissions – when setting up a new site the director permissions are normally set quite low. Change them to 755 (WordPress standard)
chown -R 755 webdir
3. Ownership – again check the ownership of the directory and update recursively if necessary:
ls -lh webdir chown -R user.group webdir
4. Hardcoded urls – depending on how your website has been coded it may contain hardcoded urls in the database. Run a find/replace on your sql dump before importing into its new home. Notepad++ is the lazy-mans way or else run from commandline.