Updated Thunderbird on the laptop this morning to the new version 11.0 and noticed Lightning had stopped working. That is normal as whatever version of Lighting is installed only works with one version of Thunderbird. Looked in the add-ons manager and noticed there was an update of Lightning ready to go, just needed to restart Thunderbird for it to install. I tried to restart Thunderbird but noticed even though I had clicked the restart it was still running in the background. After killing the process and restarting Thunderbird it acted like Lightning was not even installed (no calendar options). I checked in Thunderbird add-ons manager and noticed Lightning was listed, but the icon was missing.
I ended up going to the Lightning page on AMO, downloading and reinstalling Lightning manually. Restarted Thunderbird and this time around everything worked. Not sure, if Lightning just didn’t upgrade properly or if because Thunderbird didn’t close down gracefully or if the two were related to each other. Nonetheless, if you are having trouble getting Lightning to update in Thunderbird 11 just go to the AMO page, download and manually update.