Try cleaning the battery terminals with a can of battery cleaner. Quite often there is a poor connection between the battery terminals and the cables due to corosion. Also, check the keyswitch with a multimeter to see that it is working.
Jon, March 2013
Battery flat and/or starter solenoid contacts burnt out.
If the battery is charged and you by-pass the solenoid and it still does not start then the starter motor is the fault
