Well, I don't know the cause of this one. Restarted it this afternoon because Mail was acting wonky and Chrome was too.

Tried to restart through the Apple at top left. Menu bar, dock, and desktop items disappear, but instead of shutting down, it just hangs. So I hold down the power button for 5 seconds and it shuts down.

I wait a minute and start it back up. It gets to the white screen and that's it. Nothing

Repeated the hard shut down, same thing

I did the latest iTunes update, not sure, but other than safari update too, I think that was all. The one thing I did do differently is put it to sleep last night

I *think* that's when Mail started to act up. (showing I had email when I didn't, not quitting)

I disconnected Everything except keyboard and Ethernet and now I got to the Apple screen and it's to the desktop. Sheesh. Typing on iPhone is slow!!

So. Culprit could be USB hub, 2 external HD's or ?

Well, plugged everything back up and it's working fine. I guess if I have the problem again, I'll just plug in one at a time and boot up to see who is causing it.

Run all your utilities while it's still up.
Check your SMART status...

Any changes to the setup recently?

You can check the SMART status of your main hard drive in Disk Utility too. Click the HD icon in the upper left of DU and the SMART status is in the lower right.

Loose RAM chip or one that needs to be reset?

One of my my next suspects would be the USB hub though, historically problematic. Sometimes they just get stupid.

Or something just hiccuped.

I had a USB hub problem a while ago and decided to install an internal.
Haven't been able to sleep the machine ever since.
Icons and dock disappear, but the computer stays on with desktop pic showing.
Holding down power button shuts it down.
I have 2 external hard drives, connected, and have a feeling they are not powering down at sleepytime.
I've had more problems with USB externals this year than I have ever had with my Mac, EVER.
And it's really ticking me off.

Trust me, he's not that SMART.

I'd double and triple check your boot drive. Something isn't right. Are your apps, etc launching in a reasonable amount of time, or are they taking a bit longer? Saving and opening files going nominally? Anything that takes longer than normal when accessing the HD is a warning sign, even if the SMART check says it's OK.

Deep sleep and internal USB cards have almost never worked. Which machine? On G4s practically nothing works when a mass storage device is attached to the card.