Well this happen a lot for me. I'm developing a marvelous site and then when I try to migrate it to their real URLs, it just doesn't works. Lets see some commons errors:
Wrong .HTACCESS
Maybe we are developing in a CPANEL test account, when we try to zip it, we forget that hidden files doesn't appear and we dont select them. When we unzip the file in the server, some hosting create a fake htaccess that make us believe that we have the good one, check their date of creation to see if they are at the same date as the file index.php or similars. The solution is to check an compare both htaccess files. The error it usually give is that you can access the Home of your site but no other urls, even using the ?q=user trick.
Bad DATABASE username or Password
This will give you a blank page or a page telling you that your site have some errors. Check you PHP error logs to see what's happening.
