imac g5. won't start up. when apple logo appears at start up, the fan comes on quite strongly, and it stays in this mode was seems indefinitely, until i've heard it long enough and shut it down.

imac g5. won't start up. when apple logo appears at start up, the fan comes on quite strongly, and it stays in this mode was seems indefinitely, until i've heard it long enough and shut it down. previously, while working, the fan would come on, and i would simply re start. that eliminated the fan noise. there were also a couple of occasions when the i mac was asleep, when i attempted to wake it up, it would remain blank forcing me to re start.