Check your battery connections. If they're tight and no corrosion is present, then your battery may be dead. Recharge or jump start the battery. Still clicking? Replace the starter.
Bad battery, starting motor or both.
battery is low. get a boost or charge the battery.
Weak battery? Seized engine? Loose or corroded battery cables? Bad Starter solenoid? Bad starter? Bad neutral safety switch?
It could be a number of things. First check that the battery is fully charged.