Requested URL /index.php not found on server.

The error I’m getting is:

Not Found
The requested URL /index.php was not found on this server.

This happens when I click to any page from my homepage. The homepage is not showing up correctly either. When I’m at the homepage, I can see the generic theme design (I switched back to 2010 to troubleshoot), but none of the content that should be on my homepage except the navigation. All other pages only show this error message.

After poking around, it looks like it might be a permalink issue. But I’ve changed to the default setting for constructing permalinks and this hasn’t solved the problem. However, it does fix the homepage (shows the content I had written for it). All other pages are still not opening.

(When I was installing WP, I didn’t really know what I was doing so I had my host provider help. They didn’t seem to know what they were doing exactly, but it got things up and running so I didn’t question. It created a directory that makes my WordPress URL “/site1”. But my homepage is just normal, no slash stuff. When I log onto the server with ftp, my root folder shows a private and a public folder. The private folder has wordpress-config and the public folder has site1 which includes the wordpress-content folder. Not sure if that will help in determining a solution.)

Would appreciate help! ?? Thanks in advance!