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I don't know if this is the right forum if not can someone please direct, I have an issue with my Mac

My spec is 2.3GHz Power PC G5 Mac OSX Vers 10.4.11. 4.5 GB RAM
Over the weekend I got back from a weeks holiday and powered up my Mac. On three occasions yesterday while it was left alone or in sleep mode the fans went very loud and the screen would not come back up.
The only way to stop the fans was to hold in the start button for several seconds until it powered down and restart it up again. At one point on restart it did say I needed to restart as max os x had quit unexpectedly.

I am trying again today and working on it so far seems ok, it seems that if I leave it for a while the above will happen. Any suggestions?
Can you check the temperature with a software monitor? Also I would clean any crap out of the case and the fans.
Is that OSX the latest update for that version? If not update it
Resetting the SMU might help, I think yours is the late 2005 version How to reset the SMU on a Power Mac G5 (Late 2004) or Power Mac G5 (Late 2005)

Also try resetting the PRAM (ot might have thrown the toys out) Resetting your Mac's PRAM and NVRAM

If that doesn't work it might, I say might because the solution is scary, you might be better seeking other solutions before this if you're not very technical.

It might be that the cpu needs reseating. Which involves taking out the cpu, taking off the thermal paste and reapplying it.
The even scarier thing is you have to do it twice as it's a dual core.

I once had a graphic card that was overheating and it was because the thermal paste was badly applied, had dried and it was heating up (never buy a powercolor GFX card)
Resetting the SMU might help... Also try resetting the PRAM
Agree with both. Also, there's a bug which seems to be common involving an old forgotten print job trying for months to get itself done on an old forgotten printer - google "Mac fan noise." Though that dead screen looks like Kernel Panic due to memory seating or something.
Thanks both of you fr the advice so far I will try doing this, just out of curiosity if the mac is busy there does not seem to be a problem, i had to go out today for over six hours so I just set up a long playlist on itunes and set it to play, when I come back its still playing so no problem? It seems if I leave the mac with nothing to do the problem happens.