We replaced an older battery in our LC today as.

We replaced an older battery in our LC today as the car had difficulty starting or holding a charge. Now, nothing works. No power, no lights, no electrical of any kind. When you turn the key nothing and I mean nothing happens. What's wrong? How do we get the car going again? We bought a brand new battery but things are much worse (might as well not even call it a car) now. We checked the fuses and the seem fine.