Updating to El Capitan fails, recovery drive fails

I have an iMac from 2009. I was trying to update to El Capitan, and everything had finished downloading from the app store, it was time to restart my computer and install. When installing, I got an error, and it was unable to install El Capitan because of a corrupt file. So I clicked the "restart" button to select my original OS to boot from, but then it says that the original OS is not usable. So I am stuck in this situation, I have tried restarting the El Capitan install many times, and the same error comes up, and I still cannot boot from my original OS X.

I cannot loose information on my hard drive, so I don't know what I can do to save my computer.

Edit: When holding down option during startup, it only showed one button: install El capitan.

Message was edited by: nickrivera