It's still there. What happened just before it disappeared?

Despite all the talk that Lightning is now built into Thunderbird, it is in fact still an addon. Go to Tools|Add-ons, remove it and re-install it, using the search facility which should find the appropriate version.

Thanks, Zenos, but that didn't work.

A search for Lightning led to 'Not compatible ... [for TBird 54.0]' - but gave me the option 'Install anyway'.

That led to - 'Add-on has opted into strict compatibility checking'

(I'm actually on 54.9b1 32 bit)

Looks to me like I have to wait till someone sorts out a compatibility problem. A pity, I like Lightning - its sudden absence (unnoticed by me) led to a missed payment, and threat of legal action, so I'll be glad to see it back!

Well, you're running a pre-release beta version of Thunderbird. You may have more success if you revert to the current official release.

Perhaps you could downgrade to the release version of Thunderbird from getthunderbird.com. V54 is not a release version so a pre release version of lightning is required for it, not the one listed on the public web site.

Zenos said

Well, you're running a pre-release beta version of Thunderbird. You may have more success if you revert to the current official release.

(Thanks to Zenos and Matt - both of whom have tried to help) I have changed to Version 52.1.1 (32-bit) Thunderbird, and am told I am now on the 'Release update channel' which I take to mean the 'regular' version, not the Beta. (Not quite sure how I got onto Beta - but that's behind me now, I think). I have tried to get Lightning up and running, on V 52, but it doesn't appear, either like it was before, nor how it is supposed to look, now.

have you removed the lightning add-on and reinstalled it. That is the next step as it will not automatically do it when you "downgrade" from a prerelease version. Add-ons only know how to upgrade