Case is your LCD back light not working
Fault can be occur from below items
1) Back light faulty : change the new LCD and check
2) Back light drivers not working
* Check the FPCs connect properly
* Re solder FPC connectors and check
* Do the below path modification and check
http://www.u2ugsm.com/blog/wp-content/uploads/2014/11/Htc-one-M8-LCD-Display-Light-IC-Solution-Jumper-Problem-Ways.jpg
This path damaged due to short circuit back light
reason to burn paths
* Bad fixing LCD FPC
* Bad quality LCD