I've had this Garmin since December and all of a sudden it's not reading cadence and speed.
I've tried resetting it but I think that made it worse. Can anyone help me?
Garmin 510 uses ANT+ to communicate to the add-on devices. I would remove and re-pair the devices first to see if they are actually seeing each other first.
The speed and cadence sensor should pair as a single device and not speed and cadence separately.
I would try another speed cadence to see it works with that or try yours with another Garmin (friend?). Sometimes the sensor shifts a bit; I found that it has to be really close to the pedal to function properly.
Folks in bike shops are often very friendly/helpful with such things. The battery could also be an issue. ~510 owner.
With my ant+ sensor it got flakey then stopped working. The cause was damage to the wire between the little pedal sensor and the blockier wheel sensor - I must have caught it on my foot.
Answer Don't assume its the Garmin unit.
Do you have another ant+ device to test? I found a cheap ant+ Heart Rate Monitor is more useful than the cadence information.