In addition to HalfPint's suggestions, it may be a matter too much flour and not enough liquid to hydrate it fully. Although you can overmix bread dough, it's not usually a problem. Under-development of the gluten because it wasn't kneaded enough is more likely to happen. If the gluten chains are not well developed, the dough can't hold on to the gas produced by the yeast, and it comes out dense and heavy. Providing the recipe and telling us if you mixed this in a stand mixer or by hand could help us provide more help.
2 Comments
1. under-proofed (sp?)
2. dead yeast
if you can provide the recipe or link to it, we can better help you sort out the problem.