Pull the wheel and inspect the speedo sender
Most likely magnet is broken but may be simply that the drive ring is not engaged in the magnet - when you turn it backwards it is hooking up. (the sender will detect regardless of wheel direction)
If your magnet is whole, then check to see the drive plate is straight (it's quite soft) and when you go to re-fit the wheel, first insert the dirve into the wheel, ensuring that the 'dogs' of the drive plate are properly engaged iin the magnet slots.
If it's broken, VERY expensive replacement - there is no magnet available as individual line-item, so you must buy entore sender.
however
here is low cost solution - and you will never have that problem again!