Opera does not apply updates until you restart the browser. If it did restart either Windows rebooted or Opera crashed. If you have a browser hijacker its very likely to have crashed as they are notoriously poorly programmed and cause all sorts of problems. If Opera restarted and it didn't restore the session when it came back up than they are gone. The hijacker probably corrupted the session. Opera's current session system is only a 'continue where I left off' session. You can't load it up any other way. You should still be able to find the sites you visited in your browsers history though.
For the future you should consider installing a real session manager. There are a few for Opera and several for Chrome (which you can install in Opera with the Download Chrome Extension). I personally use Session Buddy. It can auto save sessions for just such little problems and stores them separately from Opera's more basic session system. Its a handy way to have a 'backup' of your web browsing and as we say in the industry "back it up before you jack it up."