This is usually caused by a misconfigured .htaccess file or outdated permalinks. Fix, Connect with FTP and rename the .htaccess file to .htaccess.bak, then log into the wp-admin panel and from the left hand column select Setting>Permalinks. Take note of which radio button is selected, select any other, save the changes then change it back and save again. This will create a new .htaccess file and should repair the links. If this doesn’t Check out Hard Coded Links and Search and Replace.