Gray screen, flashing folder with question mark

My macbook pro was working normal in the afternoon... I closed the computer lid and returned a couple hours later to a gray screen & flashing folder with a question mark. When I hold the option key down at startup I only get an internet recovery mode. I plugged in my airport time capsule and was able to boot from it. I used disc utility check and everything checked out ok. I used my ESET NOD32 Antivirus and it checked out ok. Finally, I went to start up disc and restarted my computer from the Mac HD. This also worked without problem. Afterwards I disconnected my airport capsule and restarted the computer as a final test. Unfortunately I am back to the flashing folder with a question mark. What do I do?