Hi,
Does it make sound on startup and does the backlight turn on ? If so, it means it's correctly starting the program, it's just the screen that doesn't works. In that case try to re-align the screen and clean the rubber contact on its back.
If it doesn't make a sound and there is no backlight, then there is no program or a corrupted program in it. Then you can try the "C button" statup to force flashing LOADER.HEX from the SD card (make sure it's still on the original SD card), or you can try to upload a program through USB. Last resort, you can re-upload the bootloader through the ICSP port using a cheap USBasp module or something similar.
All steps explained on the
Troubleshooting page of the wiki.
Good luck