No matter how prepared one is to write a book there will be places you just plain get stuck! For me it's inevitable that I'll get stuck in the middle or toward the end of the book and then it's just a long painful slog to "The End".
But, if you like to plot or pants, it doesn't matter. The way out is usually the same. Sometime the manuscript has to be set aside for awhile. Sometimes everything will click into place doing laundry, or meditating, taking a walk or watching TV. Just go way on yourself and give it some time.
My upcoming novel Tea Times Three had the most intractable middle I've ever run into. I was stuck. Stuck hard and fast for YEARS. No I'm not kidding. I wrote the beginning. Couldn't think of the middle. Decided to write the ending. Then, sort of quit. But not really. In the back of my mind for about a year and a half was "What do I do in the middle?" I didn't have an answer for the longest time, but then it came to me! At long last! And I wrote the middle and them I had to edit all the parts together. That took about seven more drafts!
This is the only book I have ever skipped the middle in order to write the ending. I've never tried it again. But there are plenty of authors who do this! If you have the urge- just write 'the good bits'. Fill in the connective tissues later.