iMac won't boot up

I have a iMac that came with 10.6 Snow Leopard. I upgraded to Snow Leopard then used the free upgrade to Yosemite. I went online last night to research something on the web and my Mac froze. Now I can't boot up. I used the button on the back to shut down but when I tried to restart, it starts to boot up but then shuts off. I tried booting off of the install DVD that came with my mac but all it will allow me to do is erase the disk. Any help would be great!

Thank you,

Davidnyc64