Welp, you may want to list the hardware.
When the HDD light does not come on, it generally means that the hard drive is not running. Now, you can ask yourself: are the fans running? if the fans are not running, you might conclude that the power supply may not be functioning.
Is the unit plugged into power? If you are sure that the unit is plugged into power, and the fans ARE running, but the HDD light never comes on, and then computer does not boot, then that is a problem.
Now one of the simplest ways to try to fix a unit that does have power, but does not boot, is to try a repair install of the XP operating system.
does the CD drawer open? do you have a CD drawer? Do you have an XP installation disk?
Read in THIS forum, sixth post from the top, to do a REPAIR installation, which will not erase your personal files.
If your only problem is that the LED doesn't come on, but otherwise it works OK, then the lead connecting the LED to the motherboard has proably come adrift. I'm assuming that's your only problem otherwise you'd be asking questions about why your computer won't boot rather than just saying that a LED doesn't come on.