This error is usually caused by Pretty Url Mod, You just begin to see a 404 Not found error when You click any Link on your site.
To fix this Problem Follow the below steps.Difficulty
First try the fix recomended in this link
If the fix doesnt work then you have to disable the mod.
Lets get started.
Login to your Control panel using any Ftp like Filezilla
Then download and upload the below attachment (repair_settings.php) into your root (www or Public/html) folder
Make sure its accessible via www.yoursite.com/repair_settings.php
Disable queryless urls and click save..
Thats all, all links would start working fine..
Now goto your package manager to Uninstall and Reinstall the mod..