1. Whatever changes you made to the httpd.conf file changed back. For example. If you made changes such for AllowOverride, Make sure you have spelled corectly either the value All or None. Use capitalization.

2. Another possiblilty is the you have created an incorrect .htaccess file. make sure you have spelled everyting correctly. Or else, try deleting the .htaccess file (back up first). A common mistake is the wrong base directory.