[Wrana]

Hmmm... If it was in Opera, it saves sessions automatically. You could either use its Recycle-bin-like button to restore tab immediately or (if you had session saved) kill it in Task Manager, reload and choose to load last active session. Now it's probably too late for either. But. You can still either try Opera's History (don't remember whether it wouldn't just load page anew... wait - there is an option in Opera's Preferences to change just that, so if you change it to load from cache before loading page from History, it should work).
Or you can just open in Explorer or your favorite shell (Far, NC, Windows Commander) one of the following folders:
Program Files\Opera\Profile\Cache4
Documents'n'Settings\your-account\Application Data\Opera\...
I recall it sometimes making profile in another subdirectory, but Cache4 should be a sure thing.
Then you just look for a html with a date/time matching that of your mistake and either open it in Opera or look with any viewer. I'd recommend the latter and if you find inside a text needed, copy it into any floder you use for such things - just in case.
Hope that's clear enough - I'm not in a particularly goof shape right now... But Opera certainly saves cache in any amount you tell it to - I made it about 100 MB and 1 month iirc.