dell chromebook 3100 2 in 1 black screen

Hi everyone, we're a big school district here in NJ and we're having an issue with these dells 3100 2 in 1 cb where all of a sudden when the student opens the cb, there's just a blank screen. I see that the cb has power because of the power light, even when i connect a charger and open the cb  the blank screen still there, I've tried power + refresh key but nothing happens, i've tried power washing it but same blank screen... This is happening all over the school district and is affecting hundreds of students... these cb are about 3 years old and they're only allowed to use it about an hour a day. Have any of you encountered this issue? Is there any other troubleshooting steps or tricks that i should perform? The cb are on version 119...thank you.