Recovering tabs that have suddenly closed without warning while working live online.

This is a recurrent problem. I'm online with many tabs open, when all of a sudden they disappear and when I try to recover them by going in "history" [using the three lines at the top right of screen], and then selecting "recently closed tabs", and then try the "restore tabs", nothing happens! Previously, this had worked to offer me a way to reopen those tabs one-by-one, but now it doesn't restore anything. ALSO, I used to be able to shut down my computer, wait a few minutes, and then restart it, and would be offered a "restore session" option when the computer restarted, but that didn't happen today even after trying this twice.

I was once shown how to restore tabs by going up to the top right of the screen, but cannot remember how to do this. Can you please help me from loosing tabs, and my mind? Thanks.