Macbook keeps booting OSX Utilities instead of Mac OS X
I had the exact same problem and my IT guy told me to reset my NVRAM. 

Just hold down ⌘ CMD+⌥ OPT+p+r on restart and keep the keys held down until it  chimes 3-4 times. Then let go. 

Worked like a charm for me. I off to back up my data now.

Apple page about resetting NVRAM.