Most likely causes for not being able to login are if cookies are blocked in some way. For example sometimes cheap or free hosting puts your site inside a frameset or frame and no cookie can be set if the out frame has a different domain than the inner one. Other possible things to block a cookie are any kind of privacy guard software and sometimes even things like google toolbar can interfere.
Something is weird in your host because I just visited your site and it ran all the db setup scripts again when I visited it and now it shows the error page.
To find out what the error is you need to look in /Data/currentlog.config file or in Web.config enable it to show errors by setting:
<customErrors mode="Off" defaultRedirect="Error.htm" />
Hope it helps,