Recently my lightning calendar tab disappeared and I can't seem to get it back, ie, I can't view the calendar in its own tab and the calendar menu item is greyed out. Other functionality is working fine, I can see the events pane on the main mail tab and they update. The only thing I've tried is reinstalling lightning but that hasn't helped.
![Image](https://farm5.staticflickr.com/4376/36939309991_28b655f45c.jpg)
Thunderbird 52.3.0 (32-bit)
Lightning 5.4
Any help much appreciated, Jamie