Why is my glass black? (Cycles)

I made a glass cup, gave it a glass shader and nothing else, kept the default settings and set the color of the glass to pure white, and it worked fine at first. Then, some time later, I noticed it was now rendering as a very dark glass, almost black but still slightly transparent. The lighting looks fine and isn’t dim. I tried looking it up and couldn’t find why it was happening.

EDIT: I figured it out. I had had clamping on (.01 or .001, i forgot, but it was the lowest possible setting) to get rid of the fireflies. I tried turning it off and the glass went back to normal.