If you can Login, try going to Settings > Site Administration > Development > Debugging -> select Developer Mode from the top most menu. and enable the option to display errors on page.
When I first went to the site and tried a few things, like going to Admin page I got this server error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, firstname.lastname@example.org and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
Which is a good thing as any of the above could be part of your problem. Searching through Server Error Logs is a good starting point.
I am not too good with server side problems so it might be better if I split this disscussion from the original post, and then Move it to Instalation Problems where Colin or Howard are more experienced and better giving you advice.