The screen is always black

[White-Tiger]

First of all Happy New Year to all friends and colleagues on this forum.
I have an All-in-One TouchSmart model 300-1125it PC.
It had an all black screen, no alarm "beep" is heard, nor flashing LEDs.
After several attempts, cleaning its interior, replacing the thermal paste, I tried resetting the BIOS using the combination of Win+B keys and the power button, as suggested by Virtual Agent on this page.
In doing so, the PC recovered and Windows 7 Home Edition started again. He also did a Windows Update.
Then, I tried to go into the BIOS to see if any important values ​​had changed with that reset, but before even changing anything, the PC displayed a completely white screen and after a few seconds it was all black again.
In practice, the PC remained active for no longer than 24 hours, but now I can no longer even reset the BIOS.
The fans start, the keyboard and mouse LEDs light up, but the PC shows no signs of life.
With a lot of effort I found this link with the Motherboard's description and I did canceled the CMOS by acting on the pins unfortunately without success. The screen still remains black.
I would like to find a schema of the components to be able to test them.
I also asked the authorized distributors for a new MB, but it is no longer available on the official channels and I cannot say how reliable the unofficial circuit like eBay is.
With the technical documentation we could also attempt a repair, is it possible that it cannot be found anywhere?
We would not like to replace the PC that is currently used to instruct processes through forms and using the touch screen.

Thanks in advance for any help and any advice.

[KrazyToad]

@White-Tiger

I have gone through your post and I appreciate all your efforts and expertise in troubleshooting this issue

At this point I suggest you contact HP Support for Service Options

Click on this link - www.hp.com/contacthp/ and select your Country to get the Contact details for HP Support

Thank you

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