If the sensor has come apart you will likely see a Tamper status reported on the Frontpoint app and website.
The sensor is designed to open and should be able to be put back together without any issues.
Close the sensor by inserting the front of the sensor into the rear cover of the Motion Sensor until you hear the Case Release Button click into place.
Ensure the sensor is closed completely and check that the Tamper status on Frontpoint app or website has disappeared.