When I try to boot my computer it displays no VGA & VGI signals, what might be the problem because I replaced the fan and motherboard

VGA is the standard way computer communicate with monitors. If your monitor is saying no signal, it means it is not detecting a signal from the computer. This could be due to a bad monitor, bad cable or bad VGA adapter.

If you change the motherboard and had on board video, then its more likely the monitor. Have you tried a different monitor?

VGA is analog video or the blue 15pin monitor connector. DVI is digital video and is larger with more pins and gives better picture quality on a monitor/PC that have DVI connectors. If after changing the motherboard you didn't reformat your hard drive (assuming the two boards are not identical), the hard drive is attempting to load the drivers for the old board and that's not good. There is also a place in the bios to tell the PC what video to attempt to load first (onboard, pci, pcie agp etc.). If you have reinstalled your OS I would guess that the bios setting is wrong as just mentioned. If it still has the old OS on it you will have to format the drive to get the new boards drivers to work correctly. Sometimes its possible to get into safe mode and copy any files or pictures you have saved before you format. But if you cannot get the monitor to load that would be a problem. Safe mode is typically accessed by hitting the F8 key repeatedly during boot up and the bios would be either the DEL key, F2, F11, or F12.

Each motherboard has a chipset that controls video, even if your using a plugin card. If you don't have the motherboards drivers installed and the old ones are still there, you will have problems. You could also buy a new hard drive to reload your OS as long as you have a retail copy of the disk, and take the old one out to use as a secondary drive later and save your data that way. The new drive would then have the new motherboard drivers on it and as long as you set that drive as the first drive in your bios boot order, it will not attempt to load the older HDD drivers when you put that one back in.