The following are a series of troubleshooting steps that should help you find the issue:
1. Check to verify that your EV3 is on. Periodically, the EV3 will shut down when not in use for a length of time.
2. Check that the port number into which you connected your Vernier Sensor Adapter matches the port number in your program.
3. Build and run a simple program that sends live readings from the sensor to the screen of the EV3.
4. Swap out sensors.
5. Try different ports (make sure you change your program and the port you are connected to).
6. Try a LEGO® sensor.
7. Restart both the MINDSTORMS® software and your EV3 brick.