Was asked this a lot on panels over the weekend, so here's a summary of things to try:
Rewind and see if the character made a decision wrong. For example, the reaction or choice was more on par with you as an individual than with the character and who they are. Sometimes following a different choice breaks things lose again. This is great for character driven work.
Reverse engineer it! This is common with TV and Movie script writers. They decided where the season should end and reverse engineer back to the starting point. This is great for plot driven work.
Skip ahead and check that you are being consistent with the rules you've laid out for the characters, plot, world, and even readers. Write down everything you do know or want in the story to make room for new ideas. It seem weird, but very helpful for first time writers.
Writer a exploratory short story or snippet. Some times writing something else or placing characters in a random situation and just writing a little something away from teh main piece can help you see things about the world and characters that might be snagging you up.
Evaluate internal and external goals, conflicts, and needs for each character and how you plan on showing this through writing or the scene. Every now again you might find you failed to establish a short term goal that creates and long term conflict or one of many combos that help drive the plot and characters.
Hope this helps some ^_^