You more than likely already have a service or application running in the background that is chatting with the scanner. No new scan jobs can be started until that conversation is complete. If waiting around does not automatically resolve the problem then try opening up Task Manager and searching through the list of processes. Shutdown anything that starts with HP by hand and then try scanning again.
Sometimes there are other services that are running from windows that do not close down properly. If you turn on/view the Command Line column in Task Manager you should be able to see extra details for these services and find any that are holding on to a scanner connection. Shutdown anything suspicious and try again.
