Today i format my "OS X El capitan" partition for installing "Yosemite". But due an error in bootable yosemite usb it didn't install on this partition. After some hours when i turned on iMac and hold down "option" key for boot into usb but just blank screen and mouse show. And every time when i turend on iMac it only show flashing folder with question mark. I've tried to resolve it by reseting "NVRAM". But after restarting problem still same. Also I've tried many "Startup key combinations for Mac" but didn't work any combination on startup. Also i have important data in my other one partition. Please can anyone tell me what i do for resolving that issue Without loosing my data?