Yosemite won't start anymore!

Hi,,

i have late 2013 iMac 27"

i tried to install elcapitan from the app store,, installation didn't complete, and after that i can't boot into Yosemite anymore!!

i tried internet recovery and i left it for 2 days and nothing has changed, so i decided to make usb installer using another mac then boot into it and try to reinstall OS X.. after i booted into the installer partition and apple logo has appeared and progress bar has completed i got this SIGN (attached in the link),, i don't know what is that and i have no idea about this error..

s31.imgup.net/IMG_2015084280.jpg

(i don't have backup and i won't do format or clean my files)