Well, constant current sources are different than say, batteries which are constant voltage sources.
Current source gives you say, 200mA to work with, and whatever you connect on it, the source will provide as much voltage as needed for the diode to use exactly 200mA.
Voltage source gives you exactly , example, 4.2 V to work with, and what ever you connect on it, it will draw as much current as it can with 4.2 V of voltage.
Now, diodes operate on constant current sources. Since batteries are not it, we have electronics circuits to regulate the current, and thus imitating the battery+driver as one constant current source.
Though, since you have already connected the diode to 12V, it is most likely dead.
Nevertheless, you should go ahead and try to construct, or buy if can't make, a driver set to some 150mA and see if the diode lights up brightly.
If it only gives you a faint glow, like a very weak LED, then it's dead.