Hi some suggestions for you to try.
1 Boot your MAC with a LiveCD/USB and select try. Then open a terminal and run this command
I have made the assumption that sda is your internal MAC hd and sdc is the external hd. You can confirm this by firing up Gparted and checking the top-right corner for the correct disk identification.
dd if=/dev/sda of-/dev/sdc
This will give you a back-up of sorts for you to work with leaving your original drive alone.
We will be working with the external drive only for now.
2 Now you have your back-up made and still have the terminal open run
Once it has installed, type and follow the prompts to check for partitions and files.
sudo apt-get install testdisk
Remember at the moment you are just checking so do not write anything to this hard drive. Any partitions found on the external drive is encouraging and we can go to suggestion 3. Nothing found on the external drive means a major disaster.
3 Assuming that the test run was successful at finding partitions on the copy, now is the time to disconnect the external drive and run testdisk on the internal drive. When the partitions have been found now is the time to allow testdisk to write a MAC-compatible boot record to the internal drive and hopefully you can now reboot into your MAC properly.
Somethings to add as explanations. The external hard drive has to be the same capacity or larger than the internal drive for the 'dd' command to work. The 'dd' command will do a byte-for-byte copy of one drive to the other. All information about testdisk can be found here. http://www.cgsecurity.org/wiki/TestDisk
Good luck. I hope this works for you.