RIP Cactus
take it out of the can and let it dry in a sieve until it starts to perk up.
Yes, likely too much water, happend to one of mine once, I just let it dry out. Maybe Google?
I took it out and will see if it dries out, but I can't say I'm terribly optimistic. The green "stem" is weepy.
Is it possible it froze? Or got sunburnt through a window? If it's overwatered and then sunburnt, the insides literally cook. Sigh. 
No, it wasn't near a window, so wasn't affected by either sun or cold. I'm pretty sure it was a water issue. I bought that whole can/stones/plants arrangement, then found that the seam of the can was facing the "front" as I wanted to position it on top of a cabinet. This was bothersome to me, so I took everything out, replaced the plastic lining that was now somewhat torn, and repotted the whole thing.
There were small plastic "stones" in the soil to promote drainage, but some of the soil was bone dry, so I added some water. I don't think I watered it again since then, but whatever the case, there was apparently too much water and nowhere for it to go in the sealed can. The other plant seems to be doing okay, though a smaller one of that same variety failed right at the beginning - it really didn't have sufficient roots to hold it in.
I think the dead cactus is a dead loss, but I'll see if I can find another one to replace it. I liked that bright yellow color.
I guess the thing to remember is that they grow in sand. Very little water retention.
Is there any hope of resurrecting my sad cactus?