some kind of an error?

Alright heres the deal...

I basically use my G5 to record music using ProTools... I dont store anything on it everything i have is stored on a Firewire external hard drive... Never had a problem untill last night... the computer just froze while i was in the middle of recording... the screen went blury (looked like plaid)... I shut it down a few times and even unpluged it for 10 minutes and i kept doing the same thing... then finally a sign poped up that said "your computer needs to shut down hold down the power for several seconds.... I did that a few time and it kept freezing on me and that sign still poped up... Also the fan came on in the inside of the computer and blew extremely fast...

Could anyone tell me what this is or what is this a sign of?

Definitely some kind of hardware problem. Logic board, graphics card, power supply, ?

Do you have your original install media? Try running the Apple Hardware Test on the machine. I don't know if it will tell you anything definitive but it's worth a try. Apple Hardware Test.

Post back with results.

Regards.