Error Loading Operating System while booting

Hello, Yesterday I got a used macbook and I was toying with hard drive partition, and while rebooting my mac today, It is giving me 'Error loading operating system' error in a black screen. I tried to reboot holding alt+s, and the only option I have is to boot into windows, Mac OS is not showing in the options list. now Im stuck, I cant boot into Windows or Mac. what am I supposed to do now ? :-(