Laserfiche Scanning will only allow 1 instance to be open at a time.  Either there is already a scanning window open (and minimized or behind another window) or there is an instance of scanning that did not properly close and the process is hung open even though the window is closed.  You can use the details or process tab of the task manager to find the open instance and close it.  I usually close the BPSessionClient, LFScan.exe, and ScanConnect or Twain sessions if present.