I suspect you have some problems between the battery and the solenoid, (located behind the battery). The main fuse is next to the solenoid, and could be the culprite, as Muay said. Poke and prod around until you figure it out.
Yep, sudden lost of power tells me it's the main fuse. And then I would want to know what caused it. And that could turn out to be alternator shorting out. So if you find it's the fuse, start checking the system.
The connector plugged into the top of the starter relay is indeed a culprit in this kind of failure as is the dog bone or fusible link in the little box beside the relay coil. Often these links get a hairline break that isn't visible and may be intermittant. Remove the link from the relay housing and take a look. If it's broken or open you can replace it with a modern automotive blade type fuse. Just get a 30A rated inline fuse holder from an auto parts store (make sure it's a 30A rated one or it may melt), install a couple of terminal eyes on the ends of the wires and screw it down where the fusible link was.