Pamela 'the explorer' Tracy here.
A few weeks ago, my family went exploring. Lately, we've found ourselves gravitating toward the Globe, Arizona, area (Yes, I'm thinking about setting a book there). This particular day, we parked the van and headed toward a waterfall that we'd see from the highway. The waterfall was cool (from a distance) but the jewel we stumbled upon was unexpected. See, we were following what we thought was a trail to the waterfall, but it was really an old road. Overgrown, broken, and thoroughly fun to explore. We picked our way past broken bottles (We were not the first to explore this road), crawled over trees stumps, and traversed mounds of dirt, and finally came to the jewel: an old bridge. Down below, the water quickly flowed and my son and his friend threw rocks and watched them splash. Me? I stood on the bridge and looked at the next place the old road would take me - right into the packed earth that lay beneath AZ 88.
We found the old road that connected Apache Junction to Globe. The new highway traveled beside it some and then was formed over it. The road we traveled abruptly ended into a wall of dirt.
Our writing is like that. Sometimes we know our story, but get excited about a side plot. We travel with our new idea - totally happy - and then hit a wall and can go no further.
Let me add to that: we can go no further at that moment.
Backtrack a little and you might find a different way to turn your story. Or, maybe it's okay to end with a road leading into a wall of dirt. My son wants to go back, not to the wall of dirt but to the stream. Maybe next time I go, my plot... I mean our adventured will follow the stream and we'll find another jewel.