The answer to this question is always “no”. Uncategorically. In some cases, there is no real problem and it is just the cement that holds the dental crown in place that has been dissolved or otherwise compromised, but the problem is, you need a trained medical professional to be able to determine why and how the dental crown fell off. But the crown may be damaged, the tooth may have decayed, and you may not be able to re-adhere the dental crown without causing damage. The best thing to do in these cases is to wait it out until you can go back to your dentist and get the dental crown put back by a professional.