One of the ways you can start or add to your story is to have somebody screw up.
Your explorers don't do the right scans and their ship sinks through the crust and gets stuck. Somebody gets into a fight with their partner over something stupid.
Human beings make mistakes. Characters need to make mistakes sometimes - and making those mistakes key to the story is one of the ways you can keep your protagonist from being a Mary Sue. How many episodes of Doctor Who hinge on the fact that the otherwise, frankly, insanely powerful and ridiculously perfect Doctor is actually a lousy pilot who's eternally messing up his navigation?
If your story's getting stagnant, think about having somebody mess something up. Maybe they said a word wrong in a spell and summoned a horde of demons instead of the helpful entity they were trying for? (You can go for horror or comedy with that. Or...any way you want to go).
Even the best of us mess up - make that true for your characters as well.