There is a partition on your hard drive called EFI. At boot up your system looks for the EFI as very first step.
EFI is responsible for the booting process, and if it is broken for some reason the computer wont boot at all.
Using your hard drive on another computer wont tell you if the EFI is still intact since it is not used in that set up. EFI is about 200 Meg in size and it is normally hidden partition. You would have to unhide it in the Disk Utility to see it.
So what can you do about the EFI?
Repair your disk using something like iPartition tool, or simular.
Do it manually: You can recreate the EFI System Partition (ESP) using the command-line gpt tool.
Or, if you have a full back up in the iCloud, you could wipe the disk and start over or run the OS X restore (use cmd+r) during boot.
Some Macs that came with OS X Snow Leopard can use Internet Recovery after installing OS X Lion or later and a software update.