won't stay on...

I have a 98 Cherrokee that i just replaced the alt because i thought that it was causing my light, gauges to come off and on. that didn't clear the problem up and now the car won't turn on. I had the battery tested off of the car and was good. i replaced the battery terminals and cleaned up the ground wire at the body. i did notice that the ground wire has a lot of corrosion under the covering but not sure if it has anything to do with why it won't start and stay on. Now my windows won't work either. I'm really hoping it's not a ECM. any help or ideas would be great.