Hello i need help! No signal

If you have a Plug in Video Card remove it and plug it back in but remember to first remove the power lead so the system has no electricity running through it.

Also what makes you think the computer is actually working? Just because lights come on and fans spin doesn't mean it's working just that there is some voltage on the rails that drive those circuits but there very easily could be no current and no voltage on the other Voltage Rails.

Have you tried plugging the monitor into a different computer and make sure it is actually working black screens can just as easily be a dead monitor and the computer is working fine. I wasted an hour or 2 doing that years ago on my test monitor and that drove me nuts when I found out the monitor had died sometime between removing the previous system and plugging in the current computer.