I don't know what changed. Perhaps it was some updates I installed on ubuntu today. Anyway, now when I restart my computer it does not show the rEFIt menu! It just goes straight to grub. Grub can't boot Mac OS X, so I have no way of booting Mac OS X now.
This is very distressing. How can I get back to Mac OS X now?
Use the Mac's built-in Startup Manager by holding the option key at startup.
http://support.apple.com/kb/HT1310
Then use the Startup Disk System Preferences panel to reselect Mac OS X as your startup volume. When Mac OS X starts up again, rEFItBlesser should automatically set rEFIt as the startup program. Then when you restart a third time, rEFIt should appear.
If rEFIt doesn't appear, then it might mean that rEFItBlesser is not properly installed. In that case, try running the rEFIt installer again.