reboot difficulties

my macbook pro has come up with a flashing folder with a question mark, ive you tubed how to rebbot my macbook pro, but it still doesnt work. when i click on disc utilities, macintosh hd does not show up. ive tried putting in my 8gb usb to download the o.s onto it but unfortunately i need an extra 4gb to continue the download until later this week, but i really need my laptop going for study. ive been trying to figure out ways for weeks. unfortunately there is no apple store in my location or near so i cant take it to a store. i dont have the disc or drive that came with it either, just the laptop and the charger. so im really doomed. whats the best way to go about this?