Before we ca give you much help, we need more information about your iMac G3.
How old is it? Do you know the Model Indentifier? If not, what color is it? Does it have a tray-loading CD drive or a slot loading CD drive? Does it have a DVD drive? How much RAM do you have? How big a hard drive do you have? Do you have the original install CDs that came with the iMac? Do you have a CD of OS 8.6 install CDs?
A few hints to the problem could be, the PRAM needs reset. The PRAM battery needs replaced. The hard drive is bad. The RAM is bad. The power supply is bad. The video board is bad. The CRT is bad.
The more info you can provide us, the more likely we can help you.
From the iMac G3 TrayLoading Manual (link at the end of this comment):
No video, the screen is either completely black, or you will see horizontal rolling lines. There will be power to the system, however, which is evidenced by an illuminated amber LED, and a running fan.
1) Reset parameter RAM. Hold down <Command> <Option> <P> <R> during startup but before “Welcome to Macintosh” appears.
2) Reset the CUDA chip and try again (see below for instructions). If the problem persists, reset the logic board. (see below for instructions)
3) Do you hear a normal boot chime at startup? Yes: Go to next step. No: Follow these steps to resolve the problem: 1. Reseat the SDRAM. 2. Reseat the processor module. 3. Replace the processor module. 4. Replace the logic board.
4) Verify that the following cables are attached securely:
• Internal RGB cable that attaches to the A/V interconnect board at J5.
• Internal video cable that attaches to the logic board at J16 and the A/V interconnect board at J2.
• Internal RGB cable that attaches to the video portion of the analog/video board at board at P301.
If the iMac will not start after step 4, replace the following items in this order, testing for start after each replaced item.
5) Replace the analog/video board. Exchange like-for-like, there are two versions of the board.
6) Replace the internal video cable that attaches to the logic board at J16 and the A/V interconnect board at J2.
7) Replace the A/V interconnect board.
8) Replace the logic board.
9) Replace the internal RGB cable that connects at the A/V interconnect board at J5 and the analog/video board at P301.
10) Replace the CRT.
The Cuda Chip
The Cuda is a microcontroller chip. Its function is to:
1) Turn system power on and off.
2) Manage system resets from various commands.
3) Maintain parameter RAM (PRAM).
4) Manage the real-time clock.
Many system problems can be resolved by resetting the Cuda chip (see the Symptom/Cure tables for examples). Press the Cuda reset button on the logic board to reset the Cuda chip. (The Cuda reset button is located on the back edge of the logic board between the modem port and the processor module. Refer to the Logic Board Diagram in the Basics chapter for location information.) If you continue to experience system problems, refer to “Resetting the Logic Board” later in this chapter.
Resetting the Logic Board
Resetting the logic board can resolve many system problems (see the Symptom/Cure tables for examples). Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules.
1) Unplug the computer.
2) Disconnect the cable that runs from the power supply to the power filter board on the bottom
side of the logic board/mass storage chassis.
3) Press the Power On button on the front of the unit.
4) Remove the logic board/mass storage chassis from the unit.
5) Remove the battery from the logic board.
6) Wait at least 10 minutes before replacing the battery.
7) Make sure the battery is installed in the correct +/- direction.
8) Reassemble the computer and test the unit.
Note: This procedure resets the computer’s PRAM. Be sure to check the computer’s time/date and other system parameter settings afterwards.
The entire iMac G3 TrayLoading Manual can be found here:
It's probably the PAV (power analog video) board for the CRT monitor gone bad. Common issue with these as they age. Not really fixable. If it has a VGA output, you might try to see if an external monitor will work. Might need a PRAM (clock) battery though, if that's never been replaced. Not sure a dead one will prevent booting in your model.