I can't reproduce this, but I just had all users locked out of Greenhopper (the menu's which are usually under the Agile tab became inaccessible). When I logged in as administrator, I was taken directly to the plugins page where Greenhopper was showing that its maintenance license had expired. Since we'd already purchased an updated license, I plugged this in and suddenly everyone had access again.
Has anyone else experience this? My understanding was that license renewals were only required for updates/maintenance, not for continued operation.