Since the tooth has had a root canal, it is fragile and prone to fracturing. You should see your dentist as soon as possible to have the cap professionally reapplied or replaced. In the meantime, you can use an over-the-counter temporary dental cement, available at most pharmacies, to temporarily secure the cap back in place. Never use household glues. If you experience mild discomfort, you can take over-the-counter pain relievers like ibuprofen. Keep the area clean by gently brushing and rinsing with warm salt water.