Before the intelligent people who know about engines start with their systematic and engineering based answers can I contribute from my long list of stupid things list:
Engine doesn't start checks!
(Not necessarily in this order)
Is the main battery on?
Is it charged?
Are the connections OK?
Has the engine fuse blown? (if so why?)
Is it in gear (start in gear protection stops everything)?
Is the kill cord connected?
Double check is the kill cord connected - I often keep mine on my keys (when going to from water) - forget to connect it - but a cursory glance looks like its there!
Beyond this its into faulty wiring issues. Need to test voltage at battery and engine with multimeter. Check that there is voltage going to starter/solenoid when key turned. Check voltage on primary coil etc.
Normally a killswitch shorts out the spark circuit. So I would stick the multimeter between the HT lead and the engine body - should be open circuit (high resistance). it is not uncommon for killswitches to fail.