Oscommerce url Re-write problem

hi, Recently, our oscommerce website has an issue with all url re-writes. All catagories and product urls are not working as they only show the contents index.php. The original ossommcere urls work, such as [login to view URL] but not the re-writes. The .htaccess code hasn't been modified. So i don't know what has caused the problem. PM me if you any questions. I need an oscommerce expert to fix this immediately.