flashing folder with question mark

My computer has a flashing folder with a question mark. I went through the Command+R and on the Utilities screen I don't see my hard drive. So I go to Disk Utility and only see "disk0" and "OS X Base System". Please HELP!!! Also, it shows that there is 40/3 MB available from 1.28GB, does that mean my stuff is still on here? How can I get my stuff off my MacBook Pro? I haven't backed it up in several weeks 😟. Also, I saw that you can back up using "time machine". How does this work so? Will it back it up automatically periodically? I need this back up to take place on its on. Thank you.