Say you have this brilliant and magnificent idea for a story, immediately you type or write away. You work like a beaver on a dam till you’re at the middle or so and then something happens. May be you feel the story is going the wrong direction, the dreaded writer's block, you let someone read it and they say it isn't so swell, or it may be you're just having a nasty hand of bad luck in life. Then for whatever reason it is, that brilliant story becomes not so brilliant. All enthusiasm is lost, you loss ambition to continue writing.
There are three ways to handle this. If it's an outside issue to fix the source of the problem, just plow through the story halfheartedly, or take a break and go on living for a bit while keeping the issue in the corner of your head. Whatever you do, do not give up, that enthusiasm will come back eventually. My rule of thumb is: finish every story you start.