imac G5 - won't boot up, strange sound

imac 2GHz PowerPC G5
1GB RAM
running OSX 10.4.11

This morning I attempted to boot up my imac G5 as I do every day. I'd just shut it down 10 hrs before, with no apparent problems. At first it seemed as though everything was normal - it made its usual humming sound and the white light (front) illuminated. Then suddenly it stopped. I tried again, it did the same thing. I continued trying for the next hour - doing things like unplugging the power source, unplugging all peripherals... throughout this it got even less responsive to my attempts, eventually not responding at all - no hum, no light, nada!
Before I really freaked out I gave it one more shot, holding the power button in for a long time...then it came alive again. It booted up strangely though, my laser on my mighty mouse lighting up for about 5 sec, then a high pitched tone that I've never heard from the computer before (like a tv's tone signal when you see the color bars) now it seems as though everything is fine.
Has anyone come across this? It's a first for me.
I'm worried that the next time I turn it off will be the last.
Should I be frantically backing up my entire system?
any help or input is appreciated.
thanks

amiga said:

Holding the power button down for a long time is something you do when you're installing a firm ware update. If you've downloaded and then restart your computer after the beep it will install the update. I've never done this without the firmware but I imagine it'd try to go through the same process.

I'm not sure what was wrong with your Mac though I'm afraid. Have you added any new hardware or software or USB peripherals recently?

Click to expand...

No new hardware, software, or peripherals...just suddenly happened.

Another update - I just returned home after an hour or so - the computer that was working is no longer. Came home to a black screen, experiencing same boot up issues.