As said, you first have to make sure you have "Retain tabs from previous session" set at the URL opera://settings/onStartup before the crash happens. Then...
If Opera starts up and you notice your tabs are missing, keep Opera open, goto the URL opera://about and take note of the "profile" path.
Then, in Windows Explorer (File Explorer), with "file extensions" and "hidden items" enabled under "View -> Show", go in your profile folder ("C:\Users\yourusename\AppData\Roaming\Opera Software\Opera Stable\Default" for example), right-click the "Sessions" folder, choose "copy", right-click on a blank spot on your desktop and "choose" paste.
Then, close Opera.
Then, delete everything in the "Sessions" folder in your profile folder. Then, in the "Sessions" folder on your desktop, select the Session_xxx file with the oldest timestamp and the Tabs_xxx file with the oldest timestamp, copy the selection and paste in the now-empty "Sessions" folder in your profile folder.
That should get you your last known good session back and therefore your tabs back.
If you close Opera before copying the "Sessions" folder though, you'll usually be out of luck though as the oldest session files will be deleted. But, you can try the method above anyway just to see.
Note though that because of issues like this, Opera has a new feature that might help recover your good session in cases like this. To enable it, you can goto the URL opera://settings/early_bird, turn on Early Bird and enable just "Emergency session restore" and restart Opera. No guarantees it will always help though.
Because open tabs and sessions are so volatile, it's a good idea to make regular backups of your "Sessions" folder (while Opera is closed) and date those backups so you know which one is the newest. You can regularly run my powershell script at https://www.reddit.com/r/operabrowser/comments/1qorutx/comment/o2ctael/ while Opera is closed to automate that if you like.
Another way to back up your open tabs is to right-click a tab, goto "Save" and choose "all tabs to a speed dial folder". Then, repeat for all other workspaces and windows. You can then rename those speed dial folders to show what workspace and window they're for and what date and time they were created. Then, if you ever need to, you can right-click one of the folders and choose "open all in tabs" to get your tabs back. This method is not tab-island and split-screen-aware though.