I have a user story that I am working. I'm realizing, four days into the sprint, that one story ought to have been decomposed further during sprint planning. I committed to having the work done by the end of the sprint, but there are roadblocks on one part of the story that mean I know it cannot be completed. It also means that I can't show the parts that are all the way done as the sprint progresses, so the scrummaster is hyperventilating that we're four days in and our burndown is nowhere near perfect.
Nothing in my training explains what to do in this situation.