The GUI does not appear on windows if the last position of the window is off screen, as would be with multiple monitors and then subsequently disconnecting one monitor. I believe this is related to #7869 and #11171. It seems that e9ff818 did not fix this problem.

This is easy to replicate; open regedit.exe (windows registry editor), find the registry keys in HKEY_CURRENT_USER\Software\Bitcoin\Bitcoin-Qt. Edit the nWindowPos entry to be a point that is off screen. The next start of Bitcoin Core will result in the window being off screen and there is no way to get it back without doing -resetguisettings