Do LED lights fail completely or gradually?

i.e are they all-or-nothing?

I've got a nice little LED flashlight that, one day, started
getting dim.

Noticed that some of the contacts were corroded and wire-brushed
the ones on the back of the emitter module. The plate that
connects the two stacks of AA batteries in the back recess of the
light's body is inaccessible so, after scratching it a little
with a long knife to no avail, I folded up some alu foil and
stuffed it down there... but still no luck.

Fooled around with it today, and couldn't even get a dim light by
applying 6v from a known good source to the feet of the emitter
module.

My first thought was that I'd hosed the LED completely by
accidentally applying reverse polarity somewhere along the line.
Possible?

If not, what about the gradual dimming? Does that fit with LED
failure, or does it narrow the cause down to some sort of
electrical contact issue?
--
PeteCresswell