This can occur for a few reasons. The most common is due to an issue with the web browser software that may cause it to close non-gracefully.
To fix this, close your browser, reopen it, then log back in. If prompted, click Reconfirm. Another issue has been identified pertaining to a bug in 1.0.x versions of Mozilla Firefox. If you are using this version (click Help > About to check), please upgrade to the most recent version of Firefox to resolve the issue.