What could cause only fans running when powering on?

I replaced the hard drive in an iMac g4. Powered on with chime several times. Before I could get the OS loaded it froze up so I powered off to restart the process. When I powered back on only the fans are running. No chime, No screen, No DVD drive working. The red light under the first panel is on. I hit the PMU button but still nothing. I replaced the 3.6V battery underneath also. Could it be the power supply unit (even tho the fans turn on) or is it something with the mother board?

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@mccracken011

Had you tried reseating the memory RAM / test with another known good one?

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No I haven't. Would that cause it to not do anything? I don't have another one on hand. It was working and booting up fine. Then nothing. Had plugged in a USB to try to load from when it froze up. Not sure that could have done anything tho.

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