While the number of sites in Sitecore increases, the number of users using Sitecore client will be huge and there will be slowness in Sitecore. And sometimes, the user session will not be available for another user to login and also there will be few issues with media library too. In such scenario, we need to close the user session explicitly by logging into admin to kick such unused sessions. The url to see all the user session (both using and used session) is
Log into to Sitecore using Admin user and kick such unused session to resolve this issue.