|Monitors are NOT "hot pluggable" !!|
DO NOT plug in or unplug a monitor's video cable into / from a video port while the computer is running. Doing so can DAMAGE the monitor's circuits and/or the video adapter's circuits.
You could try the monitor with another working computer, but you'll probably find there is nothing wrong with it, if you haven't damaged it's circuits.
No video at all on the monitor is a common symptom. Usually there's nothing wrong with your video adapter or your monitor - something else is wrong.
Are you hearing the normal beep or beeps you normally hear while the computer is booting, or no beeps at all, or a different pattern of beeps ?
Are the power led and the hard drive activity led lighting up ?
Is the hard drive activity blinking like it usually does while booting ?
Can you hear the hard drive(s) spin up, and the fans spinning ?
Failing power supplies are common. They often partially work, the leds may come on, the hard drive(s) and fans may spin, yet the computer will NOT boot all the way. You may hear NO beeps from the mboard while booting in tha case.
NOTE that if you have an emachines computer, probably the NUMBER ONE reason for a desktop emachines computer not booting normally is their BESTEC power supply is failing or is dead ! Some cheaper models of other brands also have a BESTEC power supply.
Do you have more than one video port on the computer ?
If you have a video port or ports where most of the ports are on the back of a desktop case for onboard video, and one or more video ports in slot space by itself on a video card installed in a mboard slot, if the video card is installed in a PCI-E X16 or AGP slot, in almost all cases, you will NOT get any video from the video port(s) for the onboard video - the monitor must be plugged into a video port on the card in the slot space. .
You CAN get no video from the monitor when it's plugged into the right port and the computer is working fine in Windows if the settings in Windows are not right for the monitor, and get an out of range message or similar message from the monitor itself, but you should almost always get video while booting the computer BEFORE Windows is supposed to load