I had the same problems and there are the several steps that you should do for solving your problems:
1.Add the Error reporting debug ability in settings.php by adding these lines:
These lines should be comment out after debug or after solving problems.
2.Browse the website and see what error information that you will get. If there is error with timeout or memory problem. Please change the memory_limit or max_execution_time if your hosting support to do that.
3.If there are some problems with the PHP version(There are some differences between PHP 4 and PHP 5). You will consider to upgrade or not.
4. Sometime there are several issues that are related to database or .htaccess. You'll see on the debug information and try to fix one by one