As soon as a crown becomes loose, saliva and bacteria have gotten in. Within 24 hours of exposure, decay begins (which is why we recommend brushing twice per day). The fact that is has come loose means there is a good chance there is decay present now. More concerning though is the likelihood of root fracture. Normally in situations such as this, the root has fractured and there is no way to restore it. The tooth should come out. At that point you are looking at a bridge or an implant, and usually the implant is the better choice. At a minimum, a new post and crown need to be made. A more likely scenario is that the tooth needs removal and an implant should be placed.