A few thoughts. On Windows 8.1, do you have your computer set for automatic updates? Do you have the latest update? When Windows 8.1 first came out a couple years ago, there were problems with some existing browsers. I'm sure workarounds were provided via update through Microsoft.
What version of Opera did you upgrade from? Opera 27? If it was working, can you go back to the older version, and make sure it's still working? If you're talking about Opera Presto to Opera 28, that's a more dramatic upgrade.
Finally, if you had an older version of Opera that you upgraded from, maybe you can do something with the profile. Have you saved some of your profile from the old version that was working? esp. bookmarks. If so, and if it's a version of Opera blink (Opera 27?), you could try to un-install, and reinstall the latest version of Opera. It should work with Windows 8.1. If it works, then you could try substituting a bit of the old profile in your updated Opera, to make the re-installation easier. If the version you were upgrading from was a version of Opera Presto, the profile is not compatible, other than, I would think, through the upgrade process.