monitor led is on; system led is on but still doesn't boot

[OnePablo]

does anybody know waht is the problem of my computer?
when i turned it on, the system led is on but there is nothing flashing on my screen. i have check my monitor and it is on.. i can tell that is on coz i can see that the power indicator of the monitor is on... what do you think is the problem of my computer??? plzzz post your reply..... thankzzzz...

[Sylvander]

Use these for guidance...
Sylvander’s Diagnostic Flowcharts
Download a copy of my diagnostic flowcharts from here
www.erniek.eclipse.co.uk/downloads/sylvanderdiags.zip
and print them to leaf through.
Begin on the STARTUP chart.

First [unlikely] possibility is that the PC is booting up normally but unseen, because the signal lead from the video card to the monitor has come out of the socket.
Any sign that Startup is taking place normally, but without the monitor display?
Any beeps heard?
Had the PC been working normally until this happened?
Anything changed immediately prior?

If no beeps, then you have...
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FAILURE TO COMPLETE POST, NO BEEP, NO OUTPUT SIGNAL FROM VIDEO CARD.
www.pcguide.com/vb/showthread.php?t=39685
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The usual way to deal with that is to strip back the connected hardware for a "Bare-bones boot".
Ideally the mobo would be out of the case on a non-conducting surface [to eliminate a short to the case as a possible cause].
Only things connected = PSU, mobo, CPU+fan [no video card or RAM].
If the POST runs it should test and beep because of RAM test failure.
Those beeps let you know that the POST IS running.
So you fit the video card [and connect the monitor] and see if the POST is still running and giving RAM test failure beeps.
[Remember to switch off to connect each new hardware item, then switch back on]
Then you fit the RAM and see if the POST still runs [look for blinking LED's on drives/keyboard], but gives no RAM failure beeps.
The POST should run, give a single short beep to indicate successful completion, enable the video card, and you should see a video display.
Reconnect the hardware one item at a time until POST fails to complete [no beep], and that item is causing the failure to complete the POST.