Firstly, open the y-box and check whether the screen cable is loose .
Here is a video for how to open the y-box .
Secondly, check that the plug of the screen cable has not come loose, pull it out and insert it into the slot.
Thirdly, if the fan spins , it indicates that the power supply is ok .
Lastly, if pull and insert the plug of the screen cable doesn't work, it shows that the mainboard need a replacement.