My computer was working fine just yesterday, and it was working fine earlier today as well, but after restarting it, it wouldn't turn back on.

When I press the power button, the power light is on, and the fan moves for like a second but dies again. and the computer makes the boot sound... but it just won't start.
What should I do can someone help please??

What computer is it?

The first thing to do is to open it up and remove and re-fit your ram and all cards and see if that makes any difference.

i have tried the RAM already, and nothing happened
i can't re fit my graphics card because it is built into the mother board...

If you had listed what you have already tried in your first post...

The next thing to do is check the PSU. To do this, unplug it from your mobo then short the green wire to any black wire with a paper-clip or bit of wire. If the PSU doesn't fire its fan up then it's faulty.

ok I did that,
and the fan did work..

The only things left are to remove and re-fit the CPU, applying fresh thermal grease if necessary, and checking the mobo for damaged capacitors. If capacitors are blown they often leak and/or dome the end. If you spot any caps that look bad, they will need to be replaced.

hmm ok well thanks for the help
is there any chance of giving the pc a rest for a while and that it may work again in like a weeks time or so?

That wont make any difference. One other thought. Try removing the bios battery for a minute with the power disconnected from the back just in case it's a bios error.

well the battery didn't make a difference either
oh well probably have to get a new computer
thanks for the help though :)

If you have a local computer repair shop it might well be worth getting it looked at!

Don't buy a new computer first. I think i can help you with it. Does your computer says monitor going to sleep and it turn off