If you wiggle the battery cables and see an effect - I'd say the connections to the battery are suspect (and battery cables can go open inside the terminal - replacement of the cables is the answer there).

Loosening the cable connectors and using a wire brush to clean the battery posts and terminals, then re-tightening them is the answer there.

ALSO, batteries can fail internally (kinda rare, but I have seen this). Battery replacement would be the answer there - if the terminals are clean and tight and if you have to pull HARD on the cables to make the power come back or fail, I'd suspect the battery itself.

But it's hard to tell at a distance if it's the cable connectors or battery.