Power before new battery. After new battery, NOTHING on 2008 Toyota Camry

My car was slow to start for a couple of days before it just quit starting. My brother tightened the terminals on it, and it ran for a couple more days until it quit again this morning. I still had power in the dash, headlights, etc, but my navigation wouldn't come on.
My mother went and got a new battery and installed it. Now the car has NO power. No headlights, no dash lights, no NOTHING.

What the heck happened?? Is this an easy fix? Did she just put it in wrong?

Asked by for the 2008 Toyota Camry
Sounds like one of the cables isn't tight, Double check all connections
1 more answer
Had some of the same stuff happen,Ended up being a ignition problem and a bad key.