Depending on how badly your system is infected, ComboFix may take longer to complete its routine than it normally does. However, there are circumstances ComboFix will hang or stall at various stages due to malware interference, failure to disable any other real-time protection tools
and CD Emulators
, Alchohol 120%
) so that it does not complete successfully. While that is not normal behaviour, it is not unusual. In such cases, it is helpful to know at what stage CF stalled. If you don't know and it still appears to be struck, frozen or failed to reboot, then try rebooting the computer manually.