If it can move freely it's possible that there was a short 🙁.

No necessarily VGA card, but motherboard itself. Some cases had plastic plugs instead of brass spacers + screws to hold motheboard, some used both. Maybe this case needed these plastic plugs to fix in place.

Do you have POST diagnostic card? Something like this. I find it invaluable when diagnosing these old PCs and when they remain on black screen. It's even possible that RAM got shorted and everything else is working. With card like this one you can find out at what step it stops with POST process.

Just "hanging in the air" isn't good. Take the board out of the case and have it on something non-conductive and try running it with just the CPU, Memory, and Video card. Then add in the controller cards one by one making sure everything still works.

If everything works together then you'll need to tackle mounting it into the case. Take pictures of the inside and people can recommend what to do.

Thank you for your replies!

I indeed use plastic plugs, because the brass spacers were just a bit smaller for the screws I have at home. I do not have a POST diagnostic card.

I removed all the cards except the VGA and still nothing. The heatsink on the CPU gets a little warmer after I turn on the PC but nothing happens. Also, the mobo do not beeps at all, so it seems it just gives power to the CPU, and nothing else. I will make some pics later and upload them.

If the ODD and HDD drive LEDs are constantly lit that would normally indicate a cabling error - you've got one or both plugged in the wrong way round.

VLB has a bit of a reputation for poor reliability if things aren't setup right or if the card or motherboard isn't designed well enough. So if you're using a VLB graphics card the next step would probably be to buy/borrow a plain ISA one to test with. Otherwise without a POST card or the motherboard beeping out any errors, there really isn't much to go on. Double check all jumper settings on the motherboard and VGA card (if it has any), check the motherboard for any visible signs of damage (corrosion, broken traces, etc).

POST diagnostics cards are pretty cheap on ebay if you're not in a hurry and can wait for it to be shipped from China.

Hey guys!

Sorry for disappearing, my father passed away two weeks ago, so I was not in the mood to continue this project for a time. I am grateful for your answers, I already ordered two POST diagnostic cards, just in case, and I use more plastic plugs, so the mobo does not touch the side of the case.

As it turned out today, there was nothing wrong with the mobo, though. Interestingly, my VGA was simply decided not to work with my LCD monitor. As a last resort to try something, I used a D-Sub cable to connect the PC to an older LCD TV, and it worked instantly. I could install MS-DOS 6.22 and Windows 3.11 too.

My only problem now is that I have a serial port mouse, which I cannot install or use under Windows 3.11. It works fine on my other, Windows 98 PC. I know it's off topic regarding to my inital problem, but can you help me how to make this mouse work?

I have an 8-bit SiS ISA serial/parallel card, and my IDE Controller card, which also has a serial port.

Thank you for your kind suggestions! 😀