One or more reason(s) might be applicable for you. You were idle at least for the last twenty minutes. You cleared the browser cookies very recently.
IMPRES ERP is being updated. The Web-site or Application pool was restarted. One or more Assemblies was updated. The entire application was updated. Aspnet worker process recycled causing all Session objects be disposed. Web.config file was touched by any mean(s) (using notepad, or replaced while update, build or so). Web server process was restarted. Web server was restarted.