Bread problems, diagnosed

Start with timing and feel. If the loaf crumbled on the very first slice but the crumb still feels moist, the problem is usually cohesion: the gluten network was not developed strongly enough to hold the slice together. If it is dry and sandy from the first slice, look first at hydration, excess flour, dry whole-grain additions, or overbaking. If the loaf sliced cleanly when fresh and only became crumbly a day or two later, that is a different branch: staling changes starch structure and water mobility, and reheating can temporarily refresh that loaf. This is different from dense bread: a dense loaf is heavy and tight but still cohesive; a crumbly loaf fails to hold together.

Dense bread can come from poor fermentation, insufficient dough strength, hydration or flour changes. First separate a tight but fully baked crumb from gummy underbaking, then ask whether the dough actually rose well before it went into the oven.