iMac will not start up - spinning wheel at gray screen

my intel iMac 20" (early 2006) will power on - gray screen with apple and spinning wheel for less than two minutes, then powers off.

I noticed the last 2 nights that it would not go to sleep when I chose "sleep" from the "apple" menu. This a.m., I chose the software update and restart, and the rest is history.

I'm thinking the software update may not be responsible since the "sleep"was not functioning the last two days prior to installing the software update.

I have unplugged power for 15+ seconds and unplugged all usb connections. Plugged power and usb's back in and tried to start-up with same results.

Any suggestions.

Take a look here:

http://www.mac-forums.com/forums/os-x-operating-system/93819-basic-os-x-mac-troubleshooting.html

Regards.

Try number 6 on the list first up as continuing spinning gear wheel is often a sign of PRAM problems.