I used an additional display with my laptop and moved the Android emulator there. It seems to remember the location even if the display is not connected anymore.
Is there any way to reset the position of the window so it becomes visible again?
more exactly from your home directory
(on Linux: $HOME/ on Windows: C:\Users\<your user> )
go into
.android/avd/<image name>
open the file emulator-user.ini and set window.x and window.y to 0.
window.x = 0
window.y = 0
If you don't see the hidden dir .android on Linux press Ctrl+H and from Windows select View>Options>View (Tab) and select "Show hidden files"
EDIT:
If the file emulator-user.ini doesn't exist, close the emulator window, then check the avd directory again. This is because emulator-user.ini is generated after the first run of the emulator.