Restore session shows a page with blank content (no tabs) and no active "restore" button. Restore previous history is greyed out. What is there to be done?

Firefox crashed after some computer low memory issue. When I reopened I expected to find the tabs from the previous session in the restore session tab. There is just a blank box with no tabs from the previous session (please see attached). I cannot click on the restore session button. Additionally, the restore previous session in History is greyed out. What can I do about this? I need those tabs back. Thanks!