The power supply, video card, processor and all fans are spinning, but the system does not start. What could be the cause of the problem?

Question

Iron

There was a computer at home, it was rarely used. And one fine morning, he refused to start. Literally in the evening everything was fine. At startup, the "speaker" is silent, all coolers work, but nothing is displayed on the screen. Tried to replace the power supply, RAM, battery. I tried to run only the processor, mat. board and power supply, the speaker was silent and nothing started. With all this, the bridges do not overheat, the processor works, there are no swollen capacitors and other mechanical damage on the motherboard.

Answer 1

First, reset the BIOS to default settings. To do this, you need to disconnect the power supply from the mains, remove the battery from the motherboard and wait a couple of minutes, or find the "Clear BIOS" jumper and close it. After that, try to turn on the assembly.
If that doesn't help, then remove the RAM and turn it on. If there is no squeak, then the processor is out of order, or the processor power circuit on the motherboard, or the north bridge. Check the processor on another computer, if it is working, then change the motherboard. If there is a squeak, then there are problems with the video card, or with RAM, or with the south bridge. The video card is checked by the BIOS after the RAM.
PS And be sure to check the voltage on the connected power supply. It happens that the voltage drops under load and the motherboard does not start.

Answer 2

If the screen is completely dark, then check with another monitor, try to screw the video cable, try with another port on the video card, check if you forgot to connect all the power phases to the video card, try pulling out and putting back the video card, try pulling out the video card and start with the built-in (take your time, wait a couple of minutes, it will not immediately connect).
As you understand, the problem is most likely in the video card or its environment)