I run Thunderbird as my email client and some days it refuses to start.
I’ve found that reinstalling it and then rebooting helps, but not always.
Today I think I found the real reason is something do do with the dual monitor arrangement I have.
Usually there is a second monitor attached to my laptop. When the second monitor is missing but I have not changed the setting it screws up Thunderbird.

So a simple change of screen resolution (from 3840 x 1200 to 1920 x 1200 in my case) sorts it.
Now I can stop reinstalling and rebooting.