A probable possibility is that the GPS was not active because your ephemeris data file is expired or damaged. Connect the watch with USB cable to the computer, and check for updates with Garmin Express. You need doing it regularly - GPS cannot work reliably and quickly without fresh ephemeris data. Read more about it here:
https://support.garmin.com/en-GB/?faq=CevbI1mlnV2SejYAaOQZ29