DHT11 is not reading Temperature and Humidity when Arduino is powered on using DC power Supply (12V)

I am facing one problem. I have code which read temperature and humidity using DHT11 sensor. I uploaded following code using Arduino via USB serial, I can read values of temp, humidity. Values are being read as long as Arduino is connected to same laptop via USB.

TEMPERATURE AND HUMIDITY are being read as 0 when I power on Arduino using DC12v, 700MA adapter.

I want to deploy Arduino with DHT sensors connected with it in Greenhouse to read greenhouse environmental condition but when I power on using DC adapter or battery, it is giving "0" output.

PLEASE HELP TO SOLVE THIS PROBLEM.

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  Serial.println("DHTxx test!");

  dht.begin();
}

void loop() {
  // Wait a few seconds between measurements.
  delay(2000);

  // Reading temperature or humidity takes about 250 milliseconds!
  // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
  float h = dht.readHumidity();
  // Read temperature as Celsius (the default)
  float t = dht.readTemperature();
  // Read temperature as Fahrenheit (isFahrenheit = true)
  float f = dht.readTemperature(true);