Nutshell Version: Adrenaline Shots for Plots!
Before you shove the project in a drawer forever, ask yourself these three questions, gleaned from my recent "Adrenaline Shots for Plots!" workshop and tighten up the sagging middle with an infusion of fresh new energy.
1. How can the protagonist's (hero's/heroine's) situation go from bad to worse? Can you alter the goal? Make the consequences of failure more dire? Create a ticking clock and then allow it to unexpectedly spring forward?
2. How can the antagonist (villain) be strengthened? Can s/he find the "magic bullet" the protagonists been searching for? Can s/he come into a position giving him/her power over the protagonist? Can the antagonist's motivation become more urgent?
3. How can the protagonist be handicapped? Does a friend or mentor betray him/her (or appear to)? Could a death (real/symbolic) or injury (physical/psychological) send him/ his reeling? Can the unthinkable happen?
Create a list of ideas on how you can make your protagonist's job impossible. Then write yourself into a box and dig deep to invent a new way out!