It is 10pm and you are inside in the middle of a large building. your cell phone is dead so you cannot use wifi or cell signals to find your location. unfortunately, your gps unit cannot find your location either! this is because:
your gps unit cannot send signals to the satellite when it cannot reach it by line of sight.

it is nighttime and the gps does not work without sunlight.

gps relies on the cellular network to send and receive signals.

your gps unit cannot receive satellite signals when you are indoors away from windows.

The answer is: your GPS unit cannot send signals to the satellite when it cannot reach it by line of sight.

GPS signals are based on frequencies that can be blocked by solid objects (like walls and roofs). A GPS device is using a series of satellites to detect and see where it is physically located. These frequencies are sent from these plates (satellites) and we cannot expect it to go through all kinds of barriers. When you use a GPS inside a building, a wide variety of physical barriers and potential interference sources make it difficult for the device to detect your location accurately.