Dreaded Flashing ? Please Help!

Hello-

My G5 is giving me issues and I find it very strange that is happened after my internet connection was giving me problems and my cable company came out to fix it. I believe they changed a few settings and it worked that night but the following morning.... the computer turns on and then I get a grey screen with a flashing ?!!!! HELP!!!!

I tried to insert MAC OS disk and then go to Disk Utility... but I can't go to repair disk because it says to find the folder/disk.... and I can't/don't know what to do!!

I tried to go to my local apple store and was VERY disappointed. In the past they have been SO helpful and always talk you through problems. However, this time I was told they only fix current model computers so I would just have to buy a new one. This didn't seem acceptable to me... how can someone not help diagnosis my problem.

It seems like after googling my problem it might just be the software and my computer not knowing where it is.... but I'm not too computer savvy and got stuck when trying to follow the steps.

Any help is MUCH appreciated!!!

Thank you!
Carrie