Both previous responses are good ones to check. Another thing to look at is if you are using Internet Explorer ensure that 'Friendly HTTP Error Messages' are turned off. This is done by clicking Tools, Internet Options, Advanced and deslecting Show Friendly HTTP Error Messages. This may then show you a more detailed error message.
Also I would run Services (from Start, Run) and check that all services that start 'QlikView' are running correctly.
Hope that helps,