The most common and most likely answer to this crossword clue is the 3 plus 2 plus 5 plus 4 letter word ONE OF THOSE DAYS
I've seen this crossword clue in The Guardian
O
N
E
O
F
T
H
O
S
E
D
A
Y
S
Buddy explains!
The straight part
We know that "When nothing went right" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when nothing went right = one of those days.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.