A plot can fall apart for many reasons. There could be a gaping plot hole a mile wide, like the fact your villain just ups and decides to become your main character's friend at the end. For no apparent reason. Or, those character you had planned on getting together at the end? They just don't have any chemistry. And the biggest plot problem, of course, is that you have no ending. The story just stops without a climax, no resolution. Or, even worse, you have a resolution and it sucks.
When editing, one of the first big questions you need to ask yourself is, "Does my plot make sense for my book?". In fact, this is one of the first questions I ask when I begin a full draft edit of my own work or when I'm reading a submission. I need to know if the story holds together. Because, if the story doesn't hold together, then the piece doesn't work. You might have pretty writing, engaging characters, and a great setting, but without a solid plot, your book just isn't very good.
The plot doesn't have to be the star the show, but it does have to work.
When you've read your draft and you've realized that your plot isn't holding together, you must then pinpoint exactly where it goes wrong. And this may take mulitple readings, because you're looking for something very specific.
Essentially, read the book as if it's not your own (I suggest taking a few days between readings) and read until you get bored. (You can't try to edit in this phase, because the editing will make the reading not boring.) You're looking for the part in the book that doesn't hold together, or where you lose interest. There is a good chance - not always, but there are no absolutes in life, anyways - that this is where your plot is failing you. A good plot equals a charmed reader. A bad one means a bored reader.
Next, give your book to a reader you trust. Another reader that's not you can see the plot through unbiased eyes, as they don't have the other bits of information about characters, settings, and unused backstory in their heads like you do. Essentially, your reader, if they're an honest and good reader (you perferably want someone who either is also a writer or someone is an avid reader and can be critical of your work), can tell you the plot holes and where they got bored, too.
Tell them beforehand that you don't like the plot and see what they say. Sometimes, you might need more than 1 reader to get it right. Don't be afraid of taking another person's suggestions for your plot, because while they might not be right or you don't see it that way, if you listen with an open mind, their thoughts might prompt another, even better solution.
Finally, hope to God you can come up with a solution for the problems you've found. If not, you've got some hard work ahead of you. If you and your readers find a problem, without a solution, you need to return all the way to beginning, to the basic construction of your book. Plot out the entire book again, from beginning to end, with each and every scene that's included in the book.
When I teach academic writing, I call this a reverse jot list. In creative writing, it's more like reverse pre-writing. You want to take your book apart piece by piece and see which pieces don't actually fit the puzzle but got jambed in there, anyways. Your goal is to remove any plot or scene that does not fit with the rest of the book or to add new plot elements that help explain other events or character motivations.
The other side of this is that by actually pulling apart the book piece by piece, you're going to start analyzing it in a different way. Your writing and editing brain should take over, and the possibilities will start opening to you as to ways to fix the problem. This editing task can (and should, if you're doing it right) take forever, but it's worth it if you can solve the plot problems. By analzying the parts instead of the whole (which is what reading the book does), you're going to see your book's issues in a different light.