Help, No power when turn key

At the firewall there is a fusible link with a service/safety disconnect. All of the power inside the cabin goes through that. If that connection fails completely you'll not even have a dome lamp on.
It is shown in the drawing above. Rewiring all the way to the ALT' gauge may or may not be needed. That disconnect is exposed to the elements so it does corrode. Might be the only problem.