Try holding down the Option key (or Alt if you're using a PC keyboard) when you hear the chime early in the boot process. That should produce a boot menu that will enable you to boot to OS X. You can then re-install rEFIt (or switch to rEFInd, which is my updated fork of rEFIt, since rEFIt hasn't been updated in over two years). This should fix the problem.
If using Option or Alt doesn't give you a boot menu, try using a rEFIt or rEFInd boot CD. (Both projects have them, but the rEFIt CD is more likely to work on a Mac.) Again, you might need to hold down Option or Alt to see the CD as a boot option. Once booted, you can (re-)install rEFIt or rEFInd.
If neither of these approaches work, I see three others, but they all carry some risk:
- You can move or rename the /boot/efi/efi/ubuntu/grubx64.efi file from Ubuntu. This is GRUB, and moving/renaming it will, with any luck, force the Mac's firmware to look for other boot options, and it should find the standard OS X boot loader. Once you're in OS X, you can mount the ESP ("mkdir -p /boot/efi && sudo mount_msdos /dev/disk0s1 /boot/efi"), move GRUB back to its normal location and name, and (re-)install rEFIt or rEFInd. The risk is that if the firmware can't find the OS X boot loader for some reason, your system won't boot at all until you use an emergency disc to correct matters, or maybe even until you re-install an OS.
- You can do much as above, but copy the rEFIt or rEFInd files into /boot/efi/efi/ubuntu, renaming refit.efi or refind_x64.efi to grubx64.efi. When you reboot, rEFIt or rEFInd should appear, and depending on which you use and what you do with the original grubx64.efi, you might even be able to boot both OSes right off the bat. The drawback is that subsequent GRUB updates might wipe out your changes.
- You can try using the efibootmgr program to adjust your EFI boot loader configuration to move rEFIt back to the top spot (or to give rEFInd the top spot, if you try to install it). The risk is that efibootmgr has been reported to corrupt the firmware on at least some Mac models. My impression is that this problem has been corrected with post-3.0 kernels, but I haven't found anything authoritative on this. In any event, if you try this, either read the efibootmgr man page and issue the necessary commands to reset rEFIt to be the default, or download rEFInd and use its install.sh script to install rEFInd, which will also use efibootmgr to make it the default boot loader.