Can't make rEFInd show boot menu again! I'd install rEFInd in my Mac Mini 2,1. It has a OS X Lion bootable partition and a Windows 10 install ntfs partition. I tried to boot from the ntfs partition but the Windows Installer won't run, only a black screen with a DOS cursor blinking. Now I can't make the rEFInd boot menu show up again. I turn on with "alt" key or "c" key pressed but Mac Mini ignores and (appears to) try to boot from ntfs partition again...

Any idea before I have to open the Mac Mini physically?
To get started, try to reset NVRAM:

Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. 
Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
Hold these keys until the computer restarts and you hear the startup sound for a second time.
Release the keys.

Resetting NVRAM

(If the rEFInd installation was performed  something like sudo bless --mount / Volumes / ESP --setBoot --file /Volumes/ESP/efi/refind/refind_x64.efi --shortform, and the boot volume information was recorded in NVRAM).