Macbook Pro 15 progression bar halts then circle spins
If you are able to boot from external drive, then your boot sector obviously has a problem.
Using the CMD+V key combination will boot in the verbose mode if you care to investigate and know where it hangs.
Using the Cmd + R key combination and restoring OS is the best option here.
This keyboard combination only works on Mac OSX 10.7 and later. If you want to restore the OS on earlier versions, you have to use the installation CD.