Computer is not starting

replug everything, it also helps if we have computer specs

Try replacing the motherboard battery. Obviously this might not be it, but I put a mobo in storage for a year, and had the exact same symptoms you describe. Replacing the battery solved it.

i had replugged everything.
the computer was starting earlier, it was having some boot issues. tried to correct it and now it doesn't even starts.
for the specs:
cpu: intel dual core 1.80GHz
ram: 1G DDRII
hdd: 160 GB SATA
if anythong more needed tell me i'll provide the information.

and 1 more thing, the last time it started i was trying to format the disk with my windows xp cd which had shown the blue screen of death.

when you turn it on is your PC running but only not showing on monitor or complete stop ?

when turning on the pc, the fan starts , but the beep tone or the motherboard screen(default one) is not coming. practically a complete stop.

You may need to reseat the RAM. Have you opened a computer before? You could google your MOBO number and get an error code list, but usually it is the RAM.

Anyone have a decent guide to this?

don't know what MOBO is. but had tried it. took out RAM cleaned it, and put it in another slot.

what exactly have you done to "correct it"?
if the cpu cooler is working then it might be the motherboard

this machine is my home desktop. i live in other place for studies. last time i opened it , i had broken the grub of fedora. (it was dual boot xp and fedora 13). so now i tried to fix it up by reinstalling xp. tried 3 times , but it never brought the screen after loading the necessary components. when it brought the screen i pressed the repair key and it brought the blue screen of death and after that now it doesn't starts.
problems during fixing up:
tried to install ubuntu two times but it failed at installing it in the harddisk.
tried to install xp, it restarted 3 times in midst of loading the files.

if you can access the BIOS reset it to DEFAULT
then you need to format your C and after that you should be able to install Windows XP

that's the problem. i can't access the bios. the bios screen is not coming. also i've reinserted the battery (CMOS reset).

take out the MB battery and leave it for 5 minutes
also take out the RAM and move it to a different slot
check your video card, maybe it is out of her slot

if this doesn`t work then i think you need a new MB

had replaced the MB 2 years ago. i don't think that can solve the problem. also it will be hard to find such a motherboard for dual core cpu.

power off the pc and unplug it

then you should reset bios by moving the jumper cap from pins 1-2 to pins 2-3
keep it like that for 10 seconds than move the jumper cap back to pins 1-2

see your MB manual for the BIOS pins location

If you have another HDD put it in your rig but not plug your previous then check is it giving you problem?
It might possible it causes by HDD but im not sure.
But if another HDD run up without problems then your MOBO is fine.  

gave the machine to the service center and they told that the RAM is corrupted. Gonna change the RAM. thanks for the help people.