Teeth that hurt when you bite on them can be the result of a number of causes.
In general, if you accidentally bite a hard object it is quite common for that tooth to be tender for up to a week afterward. This is similar to a sprain, where the ligament that surrounds the tooth is compressed and is sore for a few days. Avoid biting and chewing on that side for a few days or a week. Many of these injuries need no treatment if things return to normal within a week or two.
However, if the bite sensitivity is worsening,or never goes away this could indicate that you may have fractured the tooth or filling. This particularly true if you have a very sharp instantaneous pain on biting hard foods in one particular spot. This is a classic sign of a fractured tooth and it is important to get it checked before it breaks completely. This is to prevent bigger problems such as nerve involvement requiring root canal treatment, or even tooth loss.
A very common reason for bite sensitivity is when a person clenches or grinds. If one tooth hits harder than the others, after a while the continual trauma will cause that tooth to experience a dull ache, made worse when chewing or tapping the tooth. Bite irregularities can also cause the jaw joint and the muscles around the jaw to ache. First aid measures for these symptoms include moist heat to the side of the face affected and over the counter pain relievers. Avoid tapping and chewing on the side that hurts. Consult your dentist who may be able to adjust the high spot and relieve the bite discrepancy. He may also recommend a night guard.
Your dentist will check for other possible causes of bite sensitivity such as dental infection which could cause a tooth to swell up. Any persisting, worsening, or severe bite sensitivity should be checked, the sooner the better.