What does it say? Or is it blank? I suggest a pram reset to start , it may get things mostly working again.
To do a PRAM reset, you need to restart your machine and hold down these 4 keys all together at the same time.
‘Option’, ‘Command (Cloverleaf or Apple)’, ‘R’ and 'P' all together at the same time.
Keep them down until you hear 4 startup chimes, this is the sound that your computer makes when it starts up, it is like a boonngg. The screen should also cycle from white to black and then back to white along with the chimes.
The sound should also get louder.
This resets all of the system buses on your computer, USB, FireWire, display, ethernet and your keyboard/mouse/trackpad.
This will correct many faults but not necessarily yours.
Give that a try and let me know.