No bootable device -- insert boot disk and press any key um...i don't know what happened..

I just opened lid to wake up my mac then it freezed. I re-booted it and now i'm seeing "No bootable device -- insert boot disk and press any key"

i'm using intel 320 ssd 120gb. 

Should I re-install mac os again? How can I make sure that it won't happen again?
It's possible the drive is failing. Before doing anything else, backup your data. Holding the option key brings up the startup manager which shows any available volumes.

Do you have multiple drives connected to your computer? Maybe it is trying to boot using the wrong drive.