We are currently aware of an issue impacting users that perform an upgrade to Gliffy for Confluence 8.1.0. This can cause the left navigation pane to disappear.
Currently, to resolve this you will need to perform a full uninstall and reinstall of Gliffy to rectify this issue. Our engineers are working diligently to resolve this issue in an upcoming release.
Please note - this only impacts Gliffy for Confluence Server. Gliffy for Confluence Cloud users are not impacted by this issue.