Does the G5 boot from the 1.5 Install Disc?
You can slide the door down & use a straightened paperclip in the tiny hole to force the tray to open.
Tough without the Install disc, but some things to try...
Does it boot to Single User Mode, CMD+s keys at bootup, if so try...
Repeat until it shows no errors fixed.
(Space between fsck AND -fy important).
Resolve startup issues and perform disk maintenance with Disk Utility and fsck...
If worse comes to wose, you can pull the drive & use one of these...
http://www.ebay.com/itm/USB-2-0-To-SATA-IDE-3-5-2-5-Hard-Drive-Adapter-Cable/140 666951341?_trksid=p2047675.m1850&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D 1%26asc%3D11%26meid%3D8538135829043526987%26pid%3D100011%26prg%3D1005%26rk%3D1%2 6sd%3D300907250926%26
Right, after weeks of procrastination, I tried CMD+s keys at bootup but that didn't seem to work.
So, I left it a few more weeks, found the discs and turned the mac on and it was fine! So, the CMD+s must have worked after all!