You started your book. Congratulations. Your idea is flowing and chapters are coming together. Again, congrats. You have taken the steps to get an idea from your head to the page. Very nice.
Now, you are stuck. And it's more than block. You're thinking you can't finish the book. What to do? Here are five things I would recommend.
One - go back to what got you fired up about the project in the first place. Draw on that initial urge and use that energy to just 'get things done.' You can do it.
Second - use a beta reader. A fresh set of eyes looking at your writing is extremely helpful. Beta readers should be trusted friends who enjoy reading. I am betting that beta readers will give your writing a boost, give you support and offer much needed enthusiasm for your project.
Third - When you lay out your book, either in your mind or on the page, make sure you have a bold twist coming up, especially if your book is a mystery or thriller. Think of it this way, you are writing toward the next twist. The writing will come.
Fourth - set deadlines and keep them. Keep the words coming.
Fifth - set aside the time to write. Simple but you have to stay on course. Find the time. Don't get distracted, and keep writing. Finish the project.
And when it's done - let me know about it.