The system automatically logs you out after 30 minutes of inactivity (no keyboard action). To access the software again, simply log back in.
A less common reason for being sent back to the Login or Introduction page is because your internet connection has lost your 'cookie'. A cookie is an internal program run from your computer that holds a record of your login.
In either case, simply log back into the software and click on the appropriate selection in the left-hand navigation bar to go to the last page in which you were last working.