It is not uncommon but the tooth can fracture after having a post and crown. The reason it usually can't be repaired is because fracture usually is below the gums and then not enough tooth left to support new crown. You need a certain amount of tooth structure above the gums and usually the tooth fractures too far below gums to make restoring it possible.
Not if the root itself is
fractured. This usually occurs when a tooth has had a root canal and a
metal post placed in it. If the root is fractured, it will have to be
extracted and an implant is a great option for you.
It sounds as though your dentist has determined that the crown is loose because you have a fracture in the root of your tooth. Once a tooth has a root fracture it is deemed nonrestorable and will need an extraction. Typically, your best option for replacing this tooth would be by placing a dental inplant. A crown will be placed over the implant and should be very aesthetic once completed. Good luck.