Close Opera and then check the task manager. If there are any opera processes or opera update processes running, kill them. Then, start Opera and go to opera://about. Or, if you have to, restart Windows. I've had to both of those things before to get it to work. Sometimes that doesn't even help though. When that's the case, I go to http://blogs.opera.com/desktop/ and download the latest 64-bit offline installer and install to the same spot to upgrade manually. Then checking for new versions works.
So, those are some things you can try if you think it's something other than a connection issue.