Imac g5 will not boot

[Audivid]

I have an Imac g5 1.8 20 inch and my optical drive wasnt working so I was trying to boot from an external hard drive to install software and after failure to accomplish that task, my imac wouldnt boot at all!!!... I'll turn it on and the infamous apple sound will play (dunnnnnnn) and then it'll go gray and then black and then gray again and this time a GLOBE inside a gray box will appear (the mode looks almost as if i held down 'alt' after restarting) and then the globe will change to a small folder with the finder logo and a ? logo alternating inside of the folder, then the screen will go black with flashing cursors and directories in the top left of the screen. What does this mean? Can this be fixed? Thank You.

 

[Sheeds]

Try to boot off your OS X install disk. if you perform an Archive and Install it might fix it, it worked when I had a similar problem. If not then you could just reinstall OS X and start again.