IMac G3 unable to boot up

OSX isn't compatible with the older G3's.

The solution posted above is simply false. G3 iMacs can run up to OS X 10.4.
G3 iMacs need firmware updates before certain versions of OS X can be loaded. You should check the available firmware updates at the Apple site and apply any your machine is missing. This may be difficult to do if you have locked up your machine by attempting to install an OS update without the appropriate firmware update.
I feel confident I could get your machine unlocked in the shop, but I would have to try several strategies which are too complicated to describe over a low-bandwidth text channel like this.