1. Outline - or use the Snowflake MethodChances are, there's something wrong with the structure. Can't figure out what it is that's off? Go back to your outline. Don't have an outline because you like to pants? Simple - write out a small summary of each chapter until you effectively have an outline on hand.
It's worth mentioning here that many writers and authors (myself included) find the Snow Flake Method to be spectacularly helpful. In case you don't know what it is and would like to learn more, here's a handy little link.
How would time travel work? Don't ask me. I don't do Sci-Fi. But I do happen to know of a magical thing called the internet which is filled with answers to every question you have ever and will ever have. If you're struggling to move forward in your writing, there's a strong possibility that it is because you need to conduct more research first. I realized this when I was circling the drain around the issue of having one of my MC's dispose of a dead body in 1954. My guy's not a genius so I knew I couldn't have him do anything fancy like dissolving it in acid and he lives in the city so dropping the body off in the abandoned woods was a no-go as well. I felt the obvious (Law and Order) go-to was to toss the body in a garbage bag in the dumpster behind the apartment building- only plastic garbage bags weren't available at this time and neither were dumpsters. My only option was to research methods of trash disposal and find a new direction to take the story.
3. Take some time off and come back later
Okay. I'll admit it. This is actually my go-to step but there's a fine line between putting some distance between you and your writing and just procrastinating. Feel free to take some time off but if its been months since you've touched your writing ... you're probably just procrastinating. Your book won't write itself! That being said, it's perfectly natural and healthy to take some time off now and then. Let your brain work out all the problems you can't see at the moment. When you come back the mistakes and plot holes will be screaming out at you. But that's okay because now you've gained some perspective and you know what needs to be changed.
4. Have a fresh pair of eyes look it over
If you're newer to writing this one is tough with a capital T. Seriously, it can be very difficult. It's like handing your newborn baby over to a stranger - a potentially filthy, germ-ridden, disease-bearing stranger. It's hard to be vulnerable and, honestly, what's more vulnerable than letting someone take a journey into your mind? Scared yet? Don't be. Getting feedback from others is a crucial part of the process. Even before your book ever makes it to the shelf (or Amazon if you're going the self-publishing or ebook route) it'll pass through the hands of alpha readers, beta readers, editors and proofreaders.
In order to grow as a writer, you need feedback and having an honest reader help you spot the weaknesses in your writing now will pay off for you later. Does it hurt your pride? Yes. Absolutely. I've had pieces that I was convinced were brilliant only to find that everyone else saw a million flaws I couldn't. I've also had pieces I was close to just deleting forever only to discover others really connected with it. It's a mixed bag, really. You'll be both praised and criticized for things you didn't anticipate but above all, you'll gain a new understanding of both your work and other people's perception of it.
5. Read it through a different medium (kindle or print)This is especially helpful later in the game. At the moment, I'm pushing my way through another draft of my novel and I'm amazed by how differently I'm relating to the text on Kindle (and in print) than on the computer screen. If you're more old fashion and use a typewriter or pen and paper to write, try typing it up or reading it out loud.
There's no reason to rush through your writing. Take it at whatever pace you need. Whatever you do - don't give up. Whether you come back a day later or a year later, don't be afraid to finish what you started.