Mostly the Camera is connected to the logicboard (with a ribbon cable or something like that). Maybe the connection to the logicboard is canceled (normaly, this should not happen). The best option is to look into the Phone and check the connection. Use this Guide. You can see the connection to the logicboard on the last picture.
There is a way to check the power of the Mainboard. You should try to start the Gameboy without batteries, but with Power Supply. Something like this: http://www.amazon.com/Original-Game-Rechargeable-Battery-Pack-color/dp/B002QIYO56 I think the connection from the batteries to the Motherboard is crashed but I'm not sure.
You can change the screen. But it's not cheap (https://stellatech.com/microsoft-lumia-640-xl-lte-complete-front-lcd-touchscreen,a,en,819767.html) I found this, too: http://www.parts4repair.com/complete-screen-assembly-for-microsoft-lumia-640-xl/ Maybe I can create a Guide, if you buy a new one.
Check the Power cable or take another one and check it again. Maybe it's the Resetbutton, so open the Console and check this one, too. Take a Y1 (Triwing) Screwdriver to open it! https://eustore.ifixit.com/Werkzeuge/Sch...-schluessel/Tri-wing-Y1-Schraubendreher.html
open it with the Y1 Triwing screwdrivers and Check the connection of the LCD Ribbon Cable, that goes into the logic board. if the cable is connected, but the screen is still "crashed", you need a new LCD I think... I never repaired one, so the best way is to buy an old, which has an functional LCD.