when i turn my computer on the hard drive tries to boot but stops and therefore computer wont start at all, why

Laptop or Desktop?
Please give this forum the brand and model of computer, operating sytem, and age.
The cause could be almost anything, so we need your help in narrowing down the possibilities.
It can be failed hard drive, memory, power supply, cpu fan, video graphics card, cable that has vibrated loose, operating system. Spyware or other infestation. dust, dirt,hair, fiber blocking operations due to heat.
If the computer is more than three years old, any of a number of components could have failed.
But sometimes the fix is as simple as disconnecting and reconnecting all cables inside the case. Or cleaning out dust and dirt. The most common problems are failures of hard drive, memory, or video graphics.
Start with age, use, and operating system.

It is a gateway gt5656 that i bought last february so its barely a year and a half old. it has 4 gig memory and 500 hard drive, vista windows. (a 1 gig memory card was installed a month after i got it, originally 3 gig) have unplugged and replugged all the cables and now am at a loss as to what it could be