In the literal sense, this refers to something becoming untied, unwrapped, unfastened, etc.
For example: Be careful with that gift.
If it comes undone, they'll be able to see what is inside.
In the figurative sense, it means that something has either not gone as planned, or has failed in some way.
E.g.
a plan can come undone meaning it hasn't worked as expected.
A person can come undone meaning that they have lost their composure or self-control.
Andy came undone when Ann left him for Steve.
english.stackexchange.com/users/2007/Dusty
In modern English, it is the counterpart of "do up", meaning to fasten (by whatever method - tie, glue, knot, zip, velcro).
It is nearly always used literally, but when it is used of a person, it normally means that their clothing has become unfastened.
In older English, it could be used in a more abstract sense, so
"I am undone" could mean something like "my life (or my wealth, or my reputation) is ruined".
Modern readers often find the phrase amusing, because its modern meaning is so closely related to clothing.
english.stackexchange.com/users/547/Colin Fine
If a strap or a knot or some other kind of binding comes undone, it opens by accident and no longer binds anything.
If a plan or a scheme or a plot comes undone, it does the same thing.
This is a figurative sense that parallels the literal sense.
english.stackexchange.com/users/2303/Robusto
Reinforcing what Colin said, "undone" means ruined.
Two cites: And the posts of the door moved at the voice of him that cried, and the house was filled with smoke.
Then said I, Woe is me!
for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the Lord of hosts.
-- Isaiah (KJV) 6:4-5 and She's come undone
She didn't know what she was headed for
And when I found what she was headed for It was too late --
The Guess
Who, "Undun", 1969
english.stackexchange.com/users/4040/Malvolio
It's hard to explain in clear words, but the concept is clear, undo someone/thing, fall apart someone/thing.
To come undone is like saying something is finally fading away.
I think the concept would be something like "to unbecome", but fall apart is very clear.
english.stackexchange.com/users/2479/Billeeb
I've heard this phrase several times but was given several contradictory interpretations. Please provide an exact meaning of the phase.
What does "to come undone" actually mean?
english.stackexchange.com
<meaning><phrases>
english.stackexchange.com/questions/6383
english.stackexchange.com/users/2648/Denys S.
There are many different ways to use the phrase "to come undone". In the literal sense, the phrase "to come undone" refers to when something becomes untied, unwrapped or unfastened. In a more figurative sense, it means that something has either not gone as planned or has failed in some way. It can also mean that someone has lost their composure or self-control.  In modern English, it is the counterpart of "do up", meaning to fasten. Also when referring to a person, it normally means that their clothing has become unfastened.   To come undone is like saying something is finally fading away. For exaple, wealth, or reputation is ruined.
Depending on the context, to come undone can mean to become unfastened, it can refer to a plan that has not worked as expected or that has failed, or tosomething that is fading away. It can also be used for someone who has lost their composure. There are many different ways to use the phrase "to come undone". In the literal sense, the phrase "to come undone" refers to when something is no longer tied or fastened. In a more figurative sense, the phrase can be used to refer to the disappearance of something, the failure of plans or a person's loss of composure.
 In modern English, it is the counterpart of "do up", meaning to fasten. Also when referring to a person, it normally means that their clothing has become unfastened.    To come undone is like saying something is finally fading away. For exaple, wealth, or reputation is ruined.
In the literal sense, "to come undone" is used to refer to when something has become unfastened or no longer tied up. The phrase "to come undone" can be used to describe when something ceases to exist. The phrase "to come undone" can be used to describe a situation where a plan has not worked as expected or planned. The phrase "to come undone" can be used to refer to a person who has lost their composure.