You can look at the errors in the log at Administration Menu > System Log, or download the file /Data/currentlog.config
newest errors are at the bottom of the file, please don't post a lot of errors, just find the newest.
you an also see the error by doing what it says, look in Web.config and change this:
<customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
<customErrors mode="Off" defaultRedirect="mycustompage.htm"/>
Hope it helps,