It may have happened using "gdisk" on a computer with a HD and msdos-partition scheme, that the HD was converted to gpt and you may not be able to boot anymore, because the bios does not understand gpt.
1) Use the "System Rescue CD" and boot from it.
2) Use any tool (eg. gparted) to revert your HD back to the msdos-partition scheme.
3) start "testdisk" in a terminal and restore your original partitions.