After upgrading to the latest version of OS(X), rEFInd decided to disappear, leaving Sierra starting up without any bootlader prompt.
It usually happens when upgrading any of the 3 OS installed on my iMac (Linux, OSX, Windows), but every time I solved reinstalling rEFInd from OSX with the classic command:
from the rEFInd directory.
Strangely this time the commands didn’t have any effect, and the only way I had to boot Windows was pressing ALT key at boot, with no way to boot Linux.
Reading on the internet, I noticed that I was not the only one being affected by this issue (called “boot coup”), but the author was kind enough to explain how to fix (official documentation in detail).
Assuming that rEFInd is installed on a separate EFI partition (standard case), eg. disk0s1, just mount it and “bless” rEFInd executable.
Create a directory where mount the EFI partition:
Mount the EFI partition (assuming the partition is correct (diskutil list is your friend)):
mount -t msdos /dev/disk0s1 /Volumes/ESP/
God bless you:
bless --mount /Volumes/ESP/ --setBoot --file /Volumes/ESP/EFI/refind/refind_x64.efi --shortfor
Reboot, and you’re probably done.