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try pressing the Windows button and the P key together until you select Duplicate or Second Screen Only, if you are able to see your broken display at all... if you cannot see it then you will need to guess how many times to press the P key to change between the different options.
If that doesn't work, then hopefully your broken screen is still giving you a partial image to allow you to check if the graphics driver needs updating, and if the external display is being detected in Device Manager.
Reply back and let me know how you get on...
The same keys should work in Windows 10 once logged in.
Can you see the old display at all?
You could also try plugging in an external keyboard.