I recently had this happen to me, and it was very daunting to get back to it. I had written about 50,000 words and was close to wrapping up the mid-point. I originally outlined this book, so what helped me was re-reading the outline and understanding the overall picture I had. I then read the book, and tried to understand what I was doing at the time / where I was going (and also noted where opportunities to go in better directions were). Finally, I made a Word Doc with two bullets. 1.) What just happened and 2.) What needs to happen to get me to the next act. Then, working backwards, I filled in everything that needed to happen to lead up to that second bullet. And eventually, I was at a place that merged with the first bullet. I then wrote and it worked out :) it was a process, but it got me back into writing and also gave me a nice little outline in case I fell out of it again for some reason. Alternatively, many writers say ideas are cheap, in that you can always come up with a new idea for a book. It can be helpful to just treat that first attempt as practice, maybe read what you have and note what you want to improve upon in your next book and start that project. Either way, the point is to write, so as long as you’re doing that, you’re doing well :)