It's not the starter solenoid. Did you find the boost switch on the dash and did holding it down help... at least dash lights? Do you have 12V on the large red cables in the front run box... outside below your feet?
Near the engine batteries you should have a battery disconnect switch. Do you have 12V on both sides of this switch. If not, does manually turning it ON/Off several times make any difference? All the way CW (to the right) is on.
What if anything did you work on between it working and not working?
