I'm sorry to hear this occured. I hope this helps: https://support.mozilla.org/en-US/kb/how-restore-browsing-session-backup#w_check-aboutsessionrestore

Try opening recently closed windows.

Thanks.

The tabs weren't present in 'recently closed windows'

I tried 'how to restore' link but again, the tabs weren't there.

Any further suggestions?

If your tabs have disappeared in Firefox, it can be a frustrating experience. There could be a few reasons for this issue, ranging from accidental actions to technical glitches. Here are some steps you can take to try and restore your missing tabs:

Use Keyboard Shortcuts:

Press Ctrl + Shift + T (Windows) or Command + Shift + T (Mac) to reopen the most recently closed tab. You can use this shortcut repeatedly to restore multiple closed tabs. Check the Sidebar:

Click on the three horizontal lines in the upper-right corner to open the menu. Select "Web Developer" and then "Browser Toolbox." In the "Browser Toolbox" panel, click on the "Console" tab. Type gBrowser.selectedTab and press Enter. This should return the currently selected tab's information. If you see something other than "undefined," it means the tab still exists but might be off-screen or minimized.

Thanks. But the tabs have disappeared completely so short cuts and Web Developer tool don't help. I tried Dan's response (above) but this didn't work as the tabs weren't listed. Anyone any other ideas?

Have a look. https://support.mozilla.org/en-US/questions/1420979#answer-1597405