You might have been blissfully unaware of the fact that your dental crown was loose until it actually falls off. You'll probably become aware of the fact that it has come off your tooth as soon as it happens, and while you will need to have it reattached, it's not quite a dental emergency. You will have to manage the situation until you can get to your dentist, but you don't need to do much, other than take a few basic precautions with your now uncovered tooth.
Finding the Crown
You might actually feel the rather curious sensation of your dental crown lifting off the underlying tooth as the bonding agent finally gives up its grip. Sure, this can happen seemingly out of nowhere, but it's more likely to happen when the tooth is in action in some way, so you might be eating, or even brushing your teeth. If you can carefully extract the crown from your mouth, then do so. If you were eating, the crown might be in your food. If you have been brushing your teeth, be careful not to spit the crown into the sink. Try not to swallow the crown. It won't hurt you, but then it will be gone for good and a new one will need to be made.
Storing the Crown
When a natural tooth is lost, it needs to be carefully handled and transported so that its nerve endings can be preserved, increasing the chance of a successful reattachment. This isn't the case with a dental crown, and you simply need to wrap it in a piece of tissue paper and put it into an appropriate container (something small with a sealable lid is good) to bring it to your dentist.
In the Meantime
There might be an immediate increase in sensitivity as soon as the crown has detached. This will require a modification in your eating habits until the crown has been put back into position. In order to receive the crown, the underlying tooth will have had a small amount of its surface enamel shaved off. This means that the tooth will be more susceptible to extremes in temperature, so avoid food and drinks that are particularly hot or cold (eating ice cream might be a rather unpleasant experience). Chew on the other side of your mouth to minimise the use of the affected tooth, and stick to soft foods if necessary.
You'll need to see your dentist to have the crown firmly bonded back into place, but until then, just be careful with your diet and you shouldn't have too much trouble. For more information, contact a dentist who deals with dental crowns in your area.