So how do you get back "into" your story when you've taken a long break?
Well, first let me say, that it can be a challenge -- and this is the single best reason I know for writing every day if you can. It keeps the story and characters immediate and "up front" in your mind.
However, that's not always possible, so...what do you do?
I think it's key not to consider it a fault or sin or problem if you have to take a break (or just want to). Life can't be commanded, and sometimes, if you've been really busy, your brain needs a little R and R. Nothing wrong with that, and having pushed myself excessively at one point, I can tell you that for me, that was totally counter-productive. I need my breaks.
So how do I get back into my story? I read what I've written. Today, since it's been a month since I read the finished book, I started with Chapter One. If I'm in the process of writing a book, and depending how long the break was, I may start at the beginning, or I may go back a few chapters, or perhaps just a few scenes. But I read what I've written to see where I was going.
Yes, this can take some time, and yes, I will make changes to what I'm rereading -- but I don't think that's a problem, either. I think it's better to take the time and make the changes; my story will be the stronger for it.
But let's say you've only got an hour to work on your book and if you go back and read, there goes that hour, and you haven't got anything new written.
Yep, that's a problem. And that's why I think it is beneficial to either write at least a bit every day, or reread and revise at least a little every day, if you can. Or at least think about your story and maybe make some notes. I've gotten some great ideas about my books while washing dishes or folding laundry. Writing time does not automatically mean computer time. Thinking counts, too.
If you find that difficult (the brain can only deal with so much!), you might need to take the time before you start writing the book to make a really detailed outline -- and stick to it, or make notes on your outline as you change things.
I think it's important to realize that taking breaks isn't a big problem. That's life. Stopping and never coming back to your writing is a big problem.
In fact, taking breaks can be beneficial for a book. After a break, problems or inconsistencies you might have missed before can become apparent to you. Trust me, that's good.