I have a Lenovo ThinkPad T420 with an i5 processor and integrated graphics. I dropped the laptop and now the screen is not working. I replaced the screen and it is still not working. It shows the BIOS and then at Windows login the screen gets all static looking. But if I plug in a DVI-to-HDMI cable the laptop starts and works fine. Even the external display works and the laptop display works too. As soon as I unplug the cable the screen crashes and is all static. I cannot figure out the problem. The video works because the external screen still works. How can I fix this?