I'm with Muttley, a complete power loss is usually related to the earth lead. If it's not a loose connection on the battery itself, check where it attaches to the chassis. My Nova used to have horrible electrics, it was eventually traced to rust around the earthing point. Moving the earth lead two inches down on to clean metal cured every
problem.
Alternatively, get yourself a multimeter and see if the fuses are actually getting a 12 volt feed.
