Sometimes hitting alt + p to open a new settings tab and then hitting F11 on that tab helps.
While Opera is closed, you can delete the "Sessions" folder in "C:\Users\yourusername\AppData\Roaming\Opera Software\Opera Stable" to see if that helps. You will lose all open tabs and and your recently-closed tabs list will be cleared, but it might help.
If that doesn't work, while Opera is closed, you can open the "Preferences" file in the profile folder with JSONedit in tree mode, right-click on the root/browser/window_placement node, choose "delete selected node" and save your changes to reset Opera's stored window settings to see if it helps.
If you use multiple monitors, try playing around with moving the window to the other monitor and restarting Opera.