What is the opposite of an epiphany?
Epiphany has nothing to do with phones—it is etymologically an ‘out-showing’ or manifestation, and until recently was used primarily for the manifestation of a divine being: most often, as in the Feast of the Epiphany, the manifestation of Christ to the world at large.
The modern sense derives primarily from the work of James Joyce (though he had predecessors), who employed ‘epiphany’ to designate moments of revelation and insight, “a sudden spiritual manifestation, whether in the vulgarity of speech or of gesture or in a memorable phase of the mind itself.”
But such an epiphany need not be a happy or triumphant moment; it is just as likely to be an insight into catastrophe or horror or even mere banality.
So in the sense in which you use epiphany, epiphany is its own antonym!
Disillusion (v.): to make someone realize that something which they thought was true or good is not really true or good:
I hate to disillusion you, but I don't think she's coming back.
Disenchantment (n.) [uncountable]: disappointment with someone or something, and no longer believing that they are good:
Voters expressed growing disenchantment with the government.
Letdown (n.): an event, performance, etc. that is not as good as you expected it to be:
The end of the book was a real letdown.
Anticlimax (n.): a situation or event that does not seem exciting because it happens after something that was much better:
Going back to work after a month travelling in China was bound to be an anticlimax.
Verbmall has an interesting creation on the opposite of epiphany:
So, let me approach the question from an etymological perspective. An epiphany leads a person to a burst of internal light. We need a term to metaphorically express leading a person to a dark cave. Let’s save the epi-, meaning to, and let’s add the combining form -calyptry, from the Greek kalyptra, covered and hidden as by a veil.
Thus, we have epicalyptry [ep´-ee-cal-ip´-tree], deliberate concealment from self or resistance to insight. Spread the word, folks. Let’s get it into dictionaries.
While Urban Dictionary plumps for depiphany:
depiphany
an epiphany that has been forgotten.
"I just had the most brilliant idea ever!"
"What?"
"I forget."
"You had another depiphany?"
Although that's not exactly your interpretation.
A sinking feeling or a stomach drop is what comes to mind.
"I had a sinking feeling as I realised that I hadn't confirmed the bookings for the flight we were heading toward."
"My stomach dropped when I realised that I'd accidently deleted all my work."
A generic word or phrase, like sudden realization, will do quite nicely. You can also get physical and refer to the pit of your stomach.
If you are looking for a cutesy or humorous term, Douglas Adams defines the following in The Meaning of Liff:
ELY (n.) — The first, tiniest inkling you get that something, somewhere, has gone terribly wrong.
WEMBLEY (n.) — The hideous moment of confirmation that the disaster presaged in the ely (q.v.) has actually struck.
GODALMING (n.) — Wonderful rush of relief on discovering that the ely (q.v.) and the wembley (q.v.) were in fact false alarms.