Lightning is an add on. To check status of add ons you go to the add on tab in Thunderbird. Click Extensions. Do you see Lightning listed but disabled? Enable it. Lightning not listed? Type lightning in the search box and install it.

Thank you. Lighting was not listed. I installed add-on and it works now.

Evgeniyp said

Thunderbird does not show the calendar. Possibly was not installed. How to fix that?

Installed 52.4.0 and Lightning is disabled. Says it is incompatible. No option to upgrade it. Deleted it. Went to Find Add-ons. Search is gone. Had to use Browse all Add-ons. Says it is compaible with 52.0 to 52.* yet the icon says no (red circle.) Downloaded it anyway. No way to install it it from Add-ons. Launching downloaded XPI gets windows prompt for app to open it with. Navigate to THunderbird.exe. Adds it as an attachment to an e-mail.

Really? Really? Have you cmpletely forgotten about your users. This is a f#@king nightmare! I've been using Thunderbird for over 10 years (like version 3.0?) and I have never had so much trouble trying to do something so basic. Your so focussed on make it slick and fancy that you've made it completely unusable.

CONGRATULATIONS!!!!

To make matters worse, TB refused to upgrade itself. So I uninstalled it and reinstalled 52.9.1 and that fixed Lightning.

(I've seen too many posts about problems with Lightning not working with 60 that I'm just not going there even though is what the Thunderbird Download page gives you. Really??) 

Between this and the very poor performance (not responding) of Thunderbird of late, I'm seriously considering switching to a different mail program.

For what it's worth, I tried the Windows 10 Mail, People and Calendar apps and they are so mickey-mouse that I had no choice but to fix Thunderbird/Lightning. (No way to request Read Receipts, no way to set Reply-To on a message, couldn't add my other GMail contact books, no mail filters ...)