I only had two reviews on my fiction, so I obviously treasured them both (they were both positive and encouraging, with constructive criticism that I took to heart).

I noticed this morning that one of them has disappeared.  I'm perplexed why this might have happened.  Anyone got any ideas?

I don't know if reviews disappear if the person deletes their account. But it might also be a bug. The best way to learn all the details would be to ask the admins via support ticket. 

The reviewer most likely removed the review. Who knows why. Send a PM? 

I have seen a review I gave disappear. it was an older review from before the review standards were modified, and was positive in tenor, and not part of an agreed swap.  I didn't remove it.  I haven't researched why.  I doubt most reviewers go about removing their handiwork.

Well that is strange. @FAHyatt I'm not seeing any indication that it was removed by staff so I wonder why...

FAHyatt Wrote: I have seen a review I gave disappear.

Causes of disappearing reviews: 

1) the review was removed by the admins (not the case for FAHyatt, according to kanadaj)
2) the reviewer deleted it (obviously not the case for FAHyatt)
3) the story was removed (obviously not the case for precinctomega)
4) possibly: the reviewer deleted their account or was banned

Number three seems to be the most commen cause in my experience and I think there might be different forms of "removed": 
  3a) the author deleted their account (and all stories with it)
  3b) the author is still here but deleted the story
  3c) the story has not been deleted but is set to hidden? (or something similar, I don't know which options the authors have) 

Of the 102 reviews that I wrote so far, 8 have disappeared over time. Each time I couldn't find the story when I looked for it. Sometimes the author was still there, sometimes not. For one story, both the story and the review had disappeared but later both reappeared, with the count going down and up accordingly (the author said something about kanadaj un-deleting the story. Thanks for that, kanadaj!).

BTW, the "amount of reviews" achievement seems to treat these different ways of "story removed" differently, because when I received the "100 reviews" achievement two days ago after posting the latest one, the current count (from my profile page and the number of reviews in the list) was still at 94 and the real count (list on my computer) was already at 102. (same behavior for the "amount of comments" and "amount of review upvotes" achievements; it's probably because (c) is still in a database somewhere and (a)/(b) might not be; if kanadaj says that this is a bug then I will write a ticket for it).