I had the same(ish) issue too. But in my case it seems to be related to n external monitor that wasn't connected anymore. Yesterday I worked with an external monitor and just put the laptop in standby.
Today I was away and initially everything was working fine. The windows I had open, were all on the laptop screen. However, when opening a new Opera window, I could not see it. Only the thumbnail on the taskbar was there.
After restarting opera and restarting the laptop, things were still the same.
Changing display settings to pc-only didn't help either.

Finally back at home I connected the monitor again and there was my Opera window.

Unfortunately this won't help the people in this thread who don't use / never used a second screen.

I had the same(ish) issue too. But in my case it seems to be related to n external monitor that wasn't connected anymore. Yesterday I worked with an external monitor and just put the laptop in standby.
Today I was away and initially everything was working fine. The windows I had open, were all on the laptop screen. However, when opening a new Opera window, I could not see it. Only the thumbnail on the taskbar was there.
After restarting opera and restarting the laptop, things were still the same.
Changing display settings to pc-only didn't help either.

Finally back at home I connected the monitor again and there was my Opera window.

Unfortunately this won't help the people in this thread who don't use / never used a second screen.