Bug Editor stuck on "Initialise GI Systems"

Hi all,
while working on a project (using LTS 2020.3.16) I hit "play" from the editor, and all got stuck, just a spinning ball....
After a long wait I had to force quit the editor, and now when I reopen the project I'm stuck with the message "Initialise GI systems" and nothing happens.
Has anyone else had this issue?
Is there a reason for that?
How can I reopen my project?
I have a project folder backup, but it would be very irritating to lose a day's work.....
And this is quite scary, first time such a thing happens in Unity!