If it says 0mph while riding = It means you will need to remove speed sensor from wheel, check it moves OK, it might have water inside ? Grease it heavy and hopefully it works again.
It is possible to open it to see if anything is damaged but i warn you, it might not work again if you open it! – So be wise.
If in doubt, try to replace it and try again.
if the sensor is new , then check the plug on the harness because the speedometer gets signal directly from the sensor at the connector in your harness.
// Meditation doesn't mean you have to sit still....