I am on a septic system. Suddenly, nothing will drain and water is backing up into both toilets and bathtubs. What could be causing this?

Clogged drain or a clogged septic tank... time to call someone to have your tank pumped first... then snake the pipes if that doesn't solve it.

Let's check and see where the clog's at. Locate and open up the house cleanout. Is there standing water in the pipe? Back to you, Tom