My old iMac (screen sits on a white dome) opens with a tiny folder that flashes a question mark, instead of a login to user's desktop

The blinking question mark indicates that the firmware could not find a valid Operating System on your machine.

Your harddrive may have failed. Your filesystem may have be come corrupted.

Try these things.

-- try to boot up from the installation disc.

The startup manager will list all of your bootable partitions then give you a choice of which to boot. Hold down the option key then power on. Continue holding down the option key until you see the startup manager. This brings up the startup manager. Click on your hd or disc. Click on right arrow key.

-- Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime.

Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:

reset-nvram

set-defaults

reset-all

"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."

http://support.apple.com/kb/TS1812?viewlocale=en_US

Should the fail...

Try taking the battery out for 10 minutes. Put battery back in. Cross fingers. Power the machine back on.

How to eject a cd from the internal cd drive:

eject cd

List of devices:

devalias

List of variables:

printenv

More than you ever wanted to know about open firmware

http://www.firmworks.com/QuickRef.html