With Yosemite you should be able to hold [command] + [R] after powering up your Mac & before the startup tune is heard, that's the "bong'. This will startup your Mac on its Reserve drive which is installed when Yosemite was installed. This drive is invisible when not in use. It has a full copy of Yosemite installed on that partition, including Drive Utility. You can boot up on your reserve drive & effect any repairs to the main drive, such as reinstalling the OS