2005 imac g5 fails to start up and wont read disks

Hi i have a imac g5 iv had some past problems over heating and a currupted harddrive tho i managed to wipe and reinstall osx and eveything was fine but then my diskdrive stopped reading dvds (superdrive). thenone day while i was at work we had constant power cuts in my area and when i got home my mac was turned off and wouldent start up it goes to the white screen with the spinning wheel and the fans start up but thats it it just gets stuck there iv tryed loading the disk and wipeing but my disk drive just spits it out again. and when i try to run it in safe mode it goes to white screen the fans go mad and then it shuts its self down any help would be nice thanks very much