This is usually caused by accidentally double-clicking the "Scan Page" button, which causes the scanner driver to lock up.  To fix this issue, enter the Task Manager by using Crtl+Alt+Del, highlight ProDOC, and click the "End Process" button.  This will exit ProDOC and you will have to sign in again.

To avoid this issue from happening in the future, make sure the "Scan Page" button is only clicked once (note: it may take a few seconds before the scan window opens).  Also make sure that you do not navigate away from the scan window during the scanning process (for instance, back to GOLD to retrieve a Social Security number).  Once you start the scanning process, make sure to complete it before clicking away from the window.