You will need to change your program logic somewhat. At the very least, you need an else clause in your if loop.
Create an unsigned long variable (I will call it serTimeOut for this example.)
When you check Serial.Available()
if it is zero, save the value of the millis() counter in serTimeOut if serTimeOut is zero.
check to see if you timeout has been reached. IF so, reset the sensor.
if Serial.Available is not zero, set serTimeOut to zero.