Notebook HP ProBook 4720s - black screen issue

After regular switch off, second day I am not able to switch on this laptop. I had problem with GPU on motherboard, so I changed motherboard for another one. After 4 months of using, I have got now this problem. I tried to plug another LCD panel via D-SUB connector or LCD TV via HDMI, but still without any improvements. But now, it is seems to be a little different problem. Now it seems, that motherboard is working correctly and windows is running (LED HDD seeking). But I can&see anything on the LCD. Can you anyone help me, what I should to try to repair this laptop?