Could be the battery terminals are loose, that happened to me before. Or your battery is weak and might need replacing if it won't hold a charge. The clicking canister is your solenoid and it's not getting enough power to latch; when it doesn't latch the starter doesn't spin. You can also do the "shower of sparks" test and jump across the solenoid by shorting the heavy terminals to manually spin the starter (bypassing every safety feature so make certain that sucker is in neutral) if you think the solenoid might be defective. Most commonly this symptom is a dead or discharged battery.
