I'll base my answer on the assumption that when you say totally dead, you mean that when you turn the key, you can hear nothing at all, no clicking noises and starter motor not even trying to turn, even though it started easily prior to you switching it off and the battery is obviously not flat.
I would suggest that you first check the battery terminals are nice and tight and not corroded. Then check the wiring from the battery is not damaged and the negative wire is properly connected to the earth point on the bodywork. Then check all the fuses are ok. Next to check would be the wiring to the ignition switch, you could try shorting out the postive wires the run to and from switch to see if it kicks the starter motor into action and if it does this would suggest a faulty switch.
You would then turn your attention to the starter motor solenoid and check this.
If, however, when you turn the ignition key, you do hear a clicking noise, ignore most of above and concentrate on starter motor solenoid, as this will be the problem.