The most likely time you'll get stuck in any writing project is the middle.
The problem probably is rooted in what has gone before (or what has not gone before).
What could happen earlier that would make this moment more interesting, more challenging, more suspenseful, more funny, or more whatever you want it to be?
If you've run out of action, what problems could you plant earlier that will come alive in the middle?
If you find that you need to cut away from the action in the middle, what subplot could you start nearer the beginning, to come back to now?
Sometimes this means going right back to the characters, not only to the previous action. If you have miscast a character, fire him or her (as novelist Elmore Leonard says he does) and “hire” a new character. Or modify the one you have in order to get congruence between who they are and what they do.
For instance, if you need your protagonist to go to extreme measures in the middle of the story, you have to establish earlier what drives him or her and make use of that. Is her family the most important thing the world--something she would die for? Is avoiding humiliation the most important thing to him--so much so that he'd kill anyone who threatened to do so? Obviously the specifics depend on the story but we need to believe that your characters would really be motivated to do what you want them to.
The same is true of any changes in your character. If you need him to start getting tough in the middle, be sure that we see what causes the change.
The big secret of fixing problems is: look where the problem starts, not just where it crops up.
(There are lots more tips for writing well in my book, "Your Writing Coach," available from Amazon and other online and offline booksellers. For a free copy of my report, "Seven Things that Are Stopping You from Writing and How to Overcome Them," see here.