Hi Allen,
You would need to add a pull up resistor (something like 1K ohm) connected to +5V (or 3.3V would also work). When the switch opens the resistor will gently pull the signal up towards +5V. When the switch closes it will force the pin low. Without a pull up resistor when the switch opens the pin will "float" as an undriven signal and is likely to remain in it's previous state (which was low) and you will not see any change.
Regards,
TK