I usually connect a monitor to my laptop while I'm working so I have two screens, with the monitor set as my primary. But I've found that if my last session of Opera was displayed on my laptop (when it is the secondary monitor) and I use my laptop without the monitor, Opera will 'open' on the second screen even though my laptop screen is now the primary screen and the only one connected. As far as I can tell there is no way to actually access the Opera window without plugging in the monitor again. Opera will appear on the taskbar, but there is no way to interact with the window itself, only with the taskbar icon (unless you plug the monitor back in).