In order to maintain optimum system performance, the number of 'active' sessions is kept to the minimum, and so if the system detects that you have not been actively using the application for a considerable period of time, then your session is terminated.
You now need to send your browser to your normal login link.
If, a bad link issue, and it is reproduceable, please let us know via the 'Help/Bug' L/H Menu option so that we can fix it.