i can't reinstall Mac OS

My MacBook pro has suddenly switched off and went grey with a question marked folder. I follow the steps and click command+R and then selected the network, clicked fisk utilities and i dont see Macintosh so i click disk0s2 and then i try to erase it and it wont let me as the erase option is faded. i go to reinstall and it wont give me an option of the disk. HOW DO I SOLVE THIS ASAP