Help Problem with G3 Mac! reply asap pls

You couldn't have replaced the video card. iMac are notorious for being non-upgradable in that sense. Perhaps you meant something else? If your screen isnt coming on it sounds like you have a logic board issue, and it would likely be cheaper to buy another iMac G3 off ebay or something than try and get it running agian.

 

He didn't say it was an iMac - he said it was a G3:
http://www.apple.com/support/powermac/g3/

Does the HDD do work after the chime? I.e. does it seem to be loading the OS, but you just can't see it?
You can get troubleshooting info from Apple here:
http://docs.info.apple.com/article.html?artnum=300537

 

Which G3, beige or blue&white?

Check the lithium battery on the motherboard, if it measures less than 2.4 Vdc this would be the cause. Beyond that, If you have anything plugged into the usb ports, try unplugging it/them. Also try disconnecting any keyboard and mouse combination and restart.

 

Sorry if this sounds a bit patronising but have you checked that the problem is not in the monitor?