It could be the brake switch or the relay to the starter or the wiring in between them.
Could be anywhere in the wiring.
I'd start at the battery, then the key switch, then the start button... Keep following the voltage.
This is a different issue than your original post tho. Once you get the bike running again do a compression test. Often times a simple leaking valve will prevent it from making enough power to run under any sort of load.
Keep at it, and please keep us updated.