Q: all I get is a plain blue screen on startup. have reinstalled system software   with same result. what do I do now please?

a prolonged period of neglect

If the G4 were unplugged for a long period of time, the internal backup battery is likely dead, and that can make the computer forget where its startup volume is located. In a G4 iMac this battery is hard to replace as it requires deactivating the heat protection system to access the battery holder. If the heat system is not properly reassembled, you can fry the computer.

Best step for now is one of two:

1) If you still have the gray system disk that came with the computer, put it in the optical drive and start the computer by holding the c key until you get a desktop. You'll be offered an installer--ignore it. Rather, use the startup disk option from the installer disk's Options or Utilities menu to restart from your hard drive.

2) Sans disks, you can also use a keyboard combo to reset the parameter random access memory (PRAM). This will not erase date you have saved. Restart the computer and immediately press and hold four keys:  command, option, p, and  r. Keep holding them until you here a total of three startup chimes, then let go the keys and let booting continue.

If these fail to get it started, post back.

Whether it is a battery problem or not depends on what model of mac you have.  My machine has a bad battery & boots just fine.

You could let power run to the machine at all times.

Try holding down the option key then power on. This brings up the startup manager. Click on your hd. 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

How to eject a cd from the internal cd drive:

eject cd

List of devices:

devalias

List of variables:

printenv