The speedometer cable is a thick metal wire inside a plastic tube. It's entirely mechanical (not electrical at all). Sometimes the wire breaks inside the tubing, but most often it just becomes disconnected at one end or the other. It usually clips/snaps into place at the wheel, and screws into place at the back of the speedometer.
Have a look and see if it's obviously disconnected. If not, unscrew it from the speedometer and then spin the front wheel to see if the cable spins inside. If so, it's just not connected at the speedometer. If it doesn't spin, then the cable is broken or it's disconnected at the wheel, so unclip it at the wheel, and spin the cable poking out that end by hand to see if that spins the whole cable, or if the cable is broken inside. If the cable is good, then it probably wasn't fully clipped into place at the wheel, so try to fix that.
Scooters Owned: 2003 Ruckus, 2006 Ruckus, 1983 Yamaha Beluga 80, 2007 Vespa LX 150, 2009 Yamaha BWS 125, 2008 Ruckus