If you see a dentist regularly, you may assume that some dental problems can be put off until your next appointment when you might actually be facing a dental emergency that should be handled as soon as possible. If you're ever in doubt as to whether or not something is an oral health emergency, you want to err on the side of caution and see a dentist as soon as possible, but note some times when it's especially vital that you visit your dentist's office right away.
Your jaw won't open
You might have some slight jaw pain from sleeping on one side of your face or if you have a stiff neck from poor posture or physical demands at work, but if your jaw actually won't open, this can mean a tooth infection, lockjaw from tetanus, or another serious health problem. You can try a warm compress to see if this relaxes the facial muscles and alleviates the pain, but if your jaw is still locked or it's very painful to open it, don't assume this will ease up as the day goes on; see an emergency dentist right away instead.
A tooth is cracked up the middle
When a tooth is cracked up the middle, this can mean that it's so weakened that it's ready to break. If this happens, you may actually lose the tooth and need to have it extracted and replaced with a dental implant of some sort. However, a dentist may be able to fill in a minor crack or other damage with a bonding agent or put a veneer over the front before the tooth suffers more damage. This can save the tooth and mean far less work and time in the dental chair for you.
You've lost a filling
A filling doesn't just cover a hole in the tooth but it also helps to hold together the tooth itself. If you lose a filling, this can mean your tooth is now weak and vulnerable to breaking and cracking; as said above, if this happens, you might lose the tooth altogether. It's not unusual for very old fillings to lose their adhesion or to break down over time so that they come loose, and you may even swallow the filling without knowing it. However, once you notice it's gone, you want to have that tooth refilled or have a cap put over it to protect it and keep the tooth from severe damage.