Do you mean the engine kill switch, or the ignition key switch? If it is the key switch - then get a can of electrical parts cleaner and spray it into the keyswitch. Disconnect the battery, then turn the key and spray into the crevices as you turn, then remove the key and spray into the keyhole a bit more. Then let it dry for 20 minutes, reconnect the battery and try starting it. If you don't want or know how to disconnect the battery - then just spray the keyswitch really good and let it air out a bit before trying to start it. You may have just gotten some crud down into the switch causing it to short. If you store it outside then it may be corroded and need to be replaced.
If you meant the kill switch on the handlebar - then you probably need to replace the switch. Take it apart and look for discoloration on the contacts.
