Problem solved! See you below:
Sometimes Joomla installations give an odd problem in Search Engine Friendly (SEF) urls. Either the site stops working totally or the 2nd level SEF links don't work! This can be either a hosting provider's problem, inability to rename htaccess.txt to .htaccess (that is hosting provider not allowing you to use .htaccess) or any other issue. Many users using Yahoo Small Business Hosting will face this issue. So the solution for this problem is:
1. In the global configuration, turn ON SEF url's, set mod_rewrite to NO.
2. Goto configuration.php and look for a value var $live_site.
3. Enter your domain address here like this:
http://www.yourdomain.comThat should solve the trouble.