A flashing folder with question mark
It would seem that the drive is no longer "blessed".

First, try booting the Mac while holding down the option key and if the drive is in the list of boot devices then selecting it once will "bless" the drive and you should be able to boot off it once again.

If that doesn't work then boot off the bootable hard drive you mentioned and run Disk Utility which can be found in /Applications/Utilities and you should be able to see the drive in the drive list. Once there you can try to first verify then repair the disk.