I had this happen on an iPhone 7 Plus that I was working on. Double Check the connections from the screen going to the motherboard. Apparently one of the connections on the screen was not seated correctly and was causing the phone not to boot.
What size is the device? If it is a nexus 7 then you can determine the two versions by looking at the back of the device for a rear camera. if it does not have one then it is the first gen if it has a rear camera then it is the second. You can also determine it by the Version of Android it runs. The first gen only goes up to Android 5.1 and i believe the second one goes up to Android 6. Correct me if i am wrong.
its possible that your back light is turning off either due to a problem with the power supply, inverter, or the lamps them selves. Try shining a flash light and see if you can see a faint image on the screen. that will tell you if it is the backlight.
you might have to use ODIN and a stock firmware file in order to flash the tablet back to stock. i have had to do this before and it is usually my first thing to do if there is a software problem that a factory reset does not fix.
If you are using a Microsoft Account then you will have to use this link to reset your password. If you cant do this then you can still access your files by plugging the hard drive into another computer and accessing them that way. If this is a local account and not a microsoft there is software that can reset the password easily.