It seems like that the backlight of the screen on your desktop PC is already faulty but please do try these troubleshooting steps that may help.
- First is to do a hard reboot, unplug the AC adapter off the PC then press and hold the power button for 20 seconds then try booting it up again.
- If it doesn't work, check if your desktop PC has a video output wherein you can connect another monitor.
- Once an external monitor is connected observe if the display will be normal or not.
- If it's going to normal, I would suggest contacting Sony for a repair specially if the PC is still under the factory warranty.
- But if the PC is already out of warranty, I would suggest opening the PC and check the ribbon cables that connect the LCD to the motherboard if it's loose or not.