imacG3 600mhz was operating OS 9.2 but then...

[Dumb Bob]

I being a complete novice with mac was poking around and I saw a file that listed various operating systems ie. 8, 9.2, 10.something or other. I clicked on the 10. OS and now... yep! It don't work at all.
I picked this thing up at a yard sale for 10 bucks, so I'm not out lot of bucks, but I got it for a friend that is even more computer illiterate than I (hard to believe, I know) So... is there any kind hearted soul that has some straight forward suggestions as to how I might fix it.
Before I goofed it up, it was shutting down after about 20 min...I took the cover off and it seemed to alleviate the problem... unless I ask the computer to do something tricky.
If you folks tell me I'm wasting my time....I WILL believe you...lol

 

[CubeHacker]

It might be helpful if you could give us more information about this "file" you clicked on. What exactly happens when you press the power button now?

 

[JediMeister]

What do you see when you start up the computer with the alt/option key held down? Does a Macintosh HD icon show up with the Finder (looks like a half gray/half blue smiling face)? Select it and click the arrow pointing to the right to proceed. What happens?