Help! G5 lockup, gray screen w/apple, endless timer & OS X downloads

This is my first time using Apple Discussions. I've had my G5 going on 5 years. I've never done any OS X updates, having rarely visited the apple website. Last week I logged onto the apple website and came across all of the OS X updates. I went to the earliest OS X update and attempted to download it. (In hindsight the download was to an earlier version than 10.3) The G5 locked up in the middle of the download. I pressed the power button on the front of the G5 to shut it down, and a message said something along the lines that if I were to power off at this time I could destablize the system. My computer was locked, and not knowing what else to do, I went ahead and shut it down.
When I attempted to reboot it, I got only a gray screen with an apple and the never-ending circular timer. We've moved twice and eventually when I locate the box that has the restore disk, I'll put it in the cd drive and life will return to normal.

Any advice would be appreciated.

Also, once I'm up and running, do I need to download the Mac OS X updates that apply to ver. 10.3.x and later, so that I'm eventually caught up with all of the updates?

This saga began when I went to the Apple website in an attempt to download a newer version of Flashplayer.

Thanks for your assistance,
Jerry