Although dental crowns might not last forever, they're definitely intended to be a long-term addition to your smile. The actual life of a dental crown can vary, but around 10 years can be expected before a replacement crown is required, and perhaps considerably longer with proper care. Although even with the maximum amount of diligence when it comes to looking after your dental crown, there are a few things that are beyond your control. If you were to have an accident that resulted in a thump to your mouth, your natural teeth could conceivably be damaged. The same goes for your dental crowns. Sure, they're strong, but they're not unbreakable, and the same goes for the dental cement that has bonded them to the tooth underneath. How urgently do you need treatment if one of your dental crowns has fallen off?
The Urgency of the Situation
Of course a dislodged or loose dental crown needs to be examined as soon as you can manage it, but the urgency of the situation depends on how sensitive the underlying tooth is in the absence of the crown, or whether you have an impending social or professional obligation where the missing crown could potentially make you feel awkward and affect your confidence (which is entirely a personal standpoint). Aesthetics aside, how can you deal with the sensitivity of the site?
In preparing the tooth for the crown, a small amount of the covering dental enamel will have been removed. This enamel should still be present, but its thickness will have been reduced, which can make the tooth in question feel rather sensitive to almost any kind of stimuli. This can include temperature (consumption of food and beverages of a particular temperature extreme), the consumption of food and beverages with a high degree of piquancy, or perhaps even the motion of your tongue in your mouth.
Combatting the Sensitivity
You can combat this sensitivity by being mindful of what you consume. In extreme cases, you might wish to apply a small amount of dental wax to the tooth in question, shielding it to a certain extent. You could even obtain a dental crown repair kit, which comes with a small amount of dental cement and an applicator, allowing you to temporarily put the crown back into place. Do not use such a kit if you have sustained a mouth injury which is still bleeding or irritated, and be mindful of any potential allergies to the dental cement (such as cements that use eugenol).
Any type of measure to reduce your sensitivity is only going to be a temporary fix, and it's still vital that you have the crown examined and reattached as soon as you're able to.