Closed Opera window with tabs, trying to get them back from other window still open

"Assuming you haven't closed and re-started Opera, there's a button at the top-right corner which used to look like a trash can, now looks like an arrow curving around an X a bin with a recycle symbol - clicking it will pop up a list of recently closed tabs and windows. Just click the item for the window you closed.

Also, in future, always quit via the menu File -> Exit, rather than closing windows :)"