Emachine light, fan and harddrive come on only.

Check ALL cables, connections, try another monitor/cable. You didn't provide the Model of eMachine? Does it have a PCI Graphics Card? Reseat Card.

If you get nothing but a blue light and no video at all it sounds like you have a bad power supply. They are easy enough to check and replace but not for a first timer. If you know someone who knows more; get a volt meter and put it on the DC scale. Unplug a power plug to a drive or find one that is not used. Measure the Yellow to black and then the red to black wires. You should get 12 & 5 volts respectively.

if its a flashing amber check power connection.

1.Cooling fans work?
2.No Signal shown on monitor?
3.Harddrive doesn't make any noise? (To explain. Harddrive is inactive. Not spinning, and the arms aren't moving back, and forth across the platters)

Suggest it's a bad power supply. A common occurrence.

Why?
1.Owner fails to keep the inside of the computer clean, and the power supply.

Dust/Dirt/Spider Webs/Lint, you name it, builds up inside. This foreign material clogs up cooling components. Specifically the Processor fan, and Heatsink, plus the power supply fan, and heatsink's inside the power supply.

Result?
Processor heats up. Takes more power to run, until eventually it turns off. (Processor's have a thermal limit. Once this thermal limit is exceeded, BIOS turns the processor off)

Power supply heats up. Fan blades and center hub collects Dust/Dirt, as well as heatsink's inside the power supply. Cooling capacity drops tremendously.

Heat = Wasted Energy. The power supply puts out more, and more power to keep up. Eventually components inside the power supply fail. They burn up. (More on this to follow)

2.Cheap power supply. Cheap construction, and cheap materials used. Pre-built computer manufacturers generally use cheaply made power supply's. Saves them money.

A.Gauge of wiring used inside is too small.
B.Electrolytic Capacitors used are of an inferior design, and of cheap quality.
C.Circuit design is below par.
D.Other components used are of low quality.

So how does a bad power supply still light-lights, and spin fans?

Bad power rail.
Personal computer power supply's put out three main voltages.
A.12 Volts
B.5 Volts
C.3.3 Volts

Electrolytic Capacitors used in a power supply are Filters. They filter the incoming AC electricity, and the rectified DC. When one or more capacitors fail, or are failing, a bad power rail results.

1.ALL the lights uses less than 1 Watt.
2.EACH fan uses 2 to 3 Watts.

A typical Processor uses 55 to 125 Watts. Depends on which processor it is.

So you see, you can have enough power to light those lights, spin fans, and even spin the harddrive motor, but not enough to turn the processor on. (Or if it does turn on, not enough to keep it running)

You can easily prove it's the power supply at fault, if you have a known, good, power supply to use for a test.
Another 300 Watt unit that has the same power cables. (Should be stated as has at LEAST the same power cables. It can have more power cables, and these just won't be used)

No processor running you get no signal to the monitor. Computer isn't working to give a signal to the monitor.

Computer is quieter than normal, because you don't hear any harddrive noise. No processor running to access the Operating System on the harddrive. (Or programs and applications)

(Windows XP is an example of an Operating System. O/S)

Just for information, this link gives details of the components inside a harddrive. Gives you more info about the Arms, and Platters I referred to above.
Just thought I would include it. It is not advertising for this website, but is included for reference.

http://computer.howstuffworks.com/hard-disk.htm

Green light indicates the monitor is getting power. Nothing coming up on the screen indicates, No Signal.

No signal from the computer. This means the computer itself isn't working.

A bad power supply in the computer can cause this. A bad power supply is the leading cause of desktop computer failure. It is replaceable, and can be done by the owner relatively easy.

You may have a computer tower that the Power light comes on, you hear fan/s spinning, (Including the Power Supply fan), and it sounds like the harddrive is running.

ALL the lights in your computer use less than 1 Watt.
EACH fan uses 2 to 3 Watts.
A harddrive sitting there spinning in an endless loop, consumes about 2 to 3 Watts.

A Processor uses 51 to 125 Watts. Depends on what Processor it is.

A weak Power Supply will have enough power for lights, fans, and that harddrive sitting there spinning.
It won't have enough power to turn on the Processor.

[No processor operating, then the harddrive sits there spinning in an endless loop, because there is no processor to find the boot sector]

For a test, connect your monitor to a know good computer. See if it works.
Now connect that other monitor to your computer.

Try changeing the RAM and if that dosent work try the processor

Looks like the lights are on? Fans spin? Sounds like the harddrives spinning? The power supply is bad. has just enough power to light lights, and spin fans, but not enough for the processor. (No beep sounds, because BIOS hasn't done a memory count. It's not going to)

ALL the lights use less than 1 Watt.
EACH fan uses 2 to 3 Watts.
The harddrive just sitting there spinning, uses about 5 watts.

Processors use 55 Watts to 125 Watts. Depends on the processor.
Not enough power to turn the processor on, the computer won't work. Computer isn't working there isn't anything to turn the monitor on. No Signal.

Remove and reinstall memory.

if your num lock key responds, you may have to reseat/change the video card (if you are sure monitor is good). else you need to replace your motherboard.
Getback if you need any clarifications

Try using another monitor that you know works. Then try another video card. The fans are running because the computer is running ---but the monitor is not. Green to Amber means monitor is not on.
A lot can go wrong with loose wires that shouldn't be loose.
But I would try this first.