If your dough turned out thin and dense rather than fluffy and soft, then it is most likely down to your yeast. The most common reasons for this include:
1. Your yeast has expired. Not only should you check the expiry date, but yeast that has been open for 3-4 months+ may have also passed its best. Always keep yeast in the fridge to maximise its lifespan.
2. You used water that was too hot or too cold. Too hot and it could kill the yeast, too cold and it may not activate it.
3. You didn't activate the yeast. Make sure to give the yeast enough time to activate when mixing with the water (usually 5-10 minutes but sometimes longer).
4. You didn't allow the dough enough time to proof.
You might also hear that sugar is required to activate the yeast. This is not strictly true; however, the sugar will cause the yeast to froth up in the liquid, which is a great visual cue as to whether the yeast is active or not.