It can happen that you're stuck at grub prompt with no boot possible (after re-installing Windows, or randomly on a mac).
There are several things to try to repair Grub:
- boot on a liveCD/DVD and re-install Grub:
- detailed tutorials here: Grub2LiveCdInstallGuide or here: https://help.ubuntu.com/community/Grub2#Reinstalling%20GRUB%202
- on a mac, if you (accidentally) installed Grub2 on the hard drive MBR and ended up with 2 linux icons (Boot linux from EFI and Boot linux from partition X) and none of them boot anymore.
- Booting on the SuperGRUB2Disk repairs the MBR grub install.
- note: after this, in the Refit menu, you might have to boot linux from EFI and not linux from partition X anymore