Some pages in EMSupply refresh automatically and others do not. If you are viewing a page that does not refresh automatically, the system may log you off after a period of inactivity. Simply log in again.
If that does not seem to be the issue, there may be a rule built into your organization's firewall that only allows users to be logged in to a secure site for a set amount of time. Your IT department may need to change this rule to make EMSupply an exception.