Q: My Mac g5 goes down when the electricity goes off and won't power back up.   When I shut down, it powers up fine.   Electricity was off today, and now it won't power up at all.

Here is one suggestion.

You may need to replace the PRAM battery.  It is a 3 volt, lithium CR2032 watch/camera battery available at stores like Walmart, Kmart, Target, or most drugstore chains.  You should be able to the replacement yourself on any G5 iMac EXCEPT the iSight models.

Here's a link to the PRAM battery:

http://eshop.macsales.com/item/NewerTech/CR2032/

Watch this video on installing RAM in the G5 iMac, as it will show you how to open the back of your G5 iMac:

http://eshop.macsales.com/installvideos/imac_g5_mem/

Good luck.