Help! My iMac is turning on but then shutting itself off before it boots!

[JellyFish]

Something is wrong with my aluminum imac. I turn it on, I see the Apple logo and then...it shuts off. It won't finish booting.

I tried starting with the Shift key but it does the same thing.

Can somebody help???

 

[gehrbox]

Something is wrong with my aluminum imac. I turn it on, I see the Apple logo and then...it shuts off. It won't finish booting.

I tried starting with the Shift key but it does the same thing.

Can somebody help???

Zap the PRAM as follows:
http://support.apple.com/kb/HT1379?viewlocale=en_US

Then perform an SMC reset as follows:
http://support.apple.com/kb/HT1543?viewlocale=en_US

 

[greentree_uk]

sounds like a hardware problem. you'll need to visit an apple service centre

 

[CWallace]

Do your fans spin really fast when it starts? Intel CPUs will power themselves off if they overheat to prevent damage.

If the PRAM and SMC resets do not fix the problem, take it in for a service review.