My pc screen is blind even if is on.

Connect the laptop via HDMI and see if you get an image - if you do, then the motherboard is fine and it is likely a bad screen - however, check the backlight fuse to be sure. Sometimes the board "passes", but the fuse blew. That said, UNLESS you service the LCD without disconnecting the battery the odds of the fuse blowing is not as high as you think it is (thankfully), and the odds it's a bad screen are higher then a bad backlight fuse. The fuse tends to be near the LCD connector - either a thing that looks like a fuse, or a 0 hm resistor used as one. Post photos of the board and we can better tell.

Without a model to reference we cannot give instructions, but generally with HP there re two to watch for: Display assembly off and ones which can be serviced in place. A lot of the EliteBook/ProBook PCs from the G4 and up models are "panel assembly off" group, while most other HP models can be done in place like the Pavilion and "3 digit" commercial PCs (ex: HP 245). The reason ProBooks and EliteBooks tend to be display off models is because the hinge cap does not allow in place servicing of the display.

Don’t panic—it sounds worse than it is. If you can see the screen faintly when looking close, it means your system is working, but your screen backlight is out.

I’ve seen this on laptops and even one desktop monitor. A repair shop can usually fix it. If it's an older device, connecting an external monitor might be a permanent workaround.