Note that Sessions folder contains two copies of files, you need to test with single pair (Session & Tabs). But most likely Opera has already ovewritten the needed one.
This has happened here too, few times already (I'm on Opera developer branch, this is often less table) - thereby my first action after any kind of crash is quick copy of Opera session files.
Opera should have some crash detection and should copy last backup into safe place after that - but because such problems are rare, they have not implemented such behavior. Few years ago they created no session backups at all, current situation is improvement from that time for sure.