Most likely, the stolen iPhone has been powered off or is unable to access the Internet, either by cellular or wireless. As such the "Last Know Location" would appear in the Find My app for up to 24 hours. After then, it will not appear again until it is powered back on and connected to the Internet.
We are only Apple-product users here. As such, we cannot assist you with this ... nor can Apple. Apple only provides the "Find My" service to let you try to locate the lost/stolen Apple device. Apple does not get involved in their recovery.