Computer not booting up

My computer not booting up, you can hear the fan going and all I see is two(2) solid lights a green light(power) and red light(hard drive). Under normal conditions red light blinks on and off. Monitor is blank.
I tried changing the monitor and got the same results.

Its a AMD K6-2 500mhz 64MB RAM.

The computer is only about months old.

What can this be?????
HelllllllP!!!!!!!!!!!!!

Sam