@nonme1 I'm in the same boat. And we're talking about 30 tabs spread over both islands and the sidebar.
This is disheartening. For me this has been happening frequently since upgrading to Opera One.

This is on windows 11.
This has thus far not happened on windows 10

Ok - this is weird. I exited opera just now on windows 11.
Then restarted.
All the tabs were back.

@saltyfox I'm in the same boat.
I'm not going to jump through hoops to reset things.
What happens with me is that I have the extra google security.
So normally it would prompt me to insert the hardware key.
On opera and only opera when prompted to insert the key I get two competing things.
One looks like it's waiting for the key to be pressed.
The other says it sent a message to my phone.