My computer is running all right but there is nothing on the monitor. Can you help me please?

Check the monitor cabling. Otherwise the likely problem is a defective video card/chipset.

Checked the monitor cable already. How can I check the video card to make sure its defective?

The only way is to swap it with another. Or to install that card in a known working system.

If you at least get the boot menu before the screen goes black you can check in the bios to see if the monitor got reset to default. Sometimes the video will work in safe mode and the bios but not full resolution If the motherboard has onboard video as well as a card, you can remove the card and hook up to the motherboard instead to see what happens. That might also get you into the bios to reset to the card again. Don't just check the I/O cord, also check that the monitors power plug is fully seated. Once I had a monitor that wiggled the power cord loose enough to still show the little green light but not enough to power the screen. Since most lcd monitors plug in Upwards into the back, they can drop loose if moved around. You can also try reseating the card but you can't reseat onboard video.

Is anything displayed on your monitor when you try starting the computer? Are there any lights that are changing color?

No please. Nothing is displayed. Just blank. I tried the monitor with another PC and it worked.

You have narrowed it down to a video card issue. Try swapping the card with a known good one or if you have onboard video just remove the card and try that.

I had a PC that failed a bios flash and the onboard video or pci-e card would not work. I got nothing. Just for the heck of it, before I trashed the board, I put and old pci card in and was then able to get to the boot screen and reflash the bios from floppy (pci must have been default). I hate auto or internet bios flashes, they've screwed me three times now. Floppy flashes have never failed me.