Which version of QlikView are you running? Is this the first time this happens to you? Disable any proxy or firewall you may have in your QlikView Server. Make sure the logon account in the service is working, just in case, log on and off with that account, then assign the logon permissions with this account in the services console again.
If you are not running QlikView Server 11 SR2 Update 2 release 11440, please update before trying anything.
Hope that helps.