No clone? No Time Machine back up? No backup of any kind? Install Yosemite from scratch. If you have files you do not wish to lose on the drive, install Yosemite on a different drive, then connect the old one and open it in finder to recover your files.
No backup of any kind. I decided to try upgrading to El Cap and switching from Chimera to Clover without any backup plan. I've had so many problems over the past few days with both El Cap and Clover (on old drives, new blank drives, etc.) that I just want to re-install Yosemite with Chimera on my old hard drive. I'd like to be able to do that without having to reinstall all of my apps & reset all Users, etc. that are already installed on that old drive. The data's all there, I just don't know if reinstalling Yosemite & Chimera over El Cap and Clover will work.