It is not unusual for a tooth that takes a hard blow, or has a bite irregularity to become loose. Also, if you have periodontal disease loss of bone can contribute to teeth becoming loose. If the tooth is very loose, shifts every time you bite, or has been loose is not getting better you should have the dentist check it. With x rays and tests he can determine what is the cause. Sometimes a simple bite adjustment can help, but other times there is a serious underlying problem which needs to be addressed.