It's a gigabyte motherboard. When I press the button, nothing comes on the screen and it beeps:
*LONG BEEP* *LONG BEEP* *short beep*
What does it mean?
You should start by having only the necessary components in (power supply, RAM, Processor, video card, and only the required fans for the video and processor). Everything else should be disconnected (all optical drives, hard drive, all other pci cards, all usb devices, just everything).
Make sure your fans are spinning. Remove all RAM Dimms, try to plug in one at a time, trying it in each RAM slot as you go. This will help determine if it is the RAM or one of the RAM slots that is causing the problem. If nothing helps, remove the video card (if you have on-board video, you can just try then, otherwise you will need to test with a replacement card). If it still won't post, there could be something wrong with the video card slot.
If you can't even get to the BIOS screen, and if you see no error message whatsoever, it is definitely a hardware issue. You may have things (memory, cards, power etc.) incorrectly connected. It can also be a faulty power supply/connection. Try rechecking all your connection, and verify your power supply is of the correct output and connected well. Failing all that, you may have ended up with a faulty MoBo.
It seems like a video adapter error at first glance. Another possibility is a RAM error.