Battery problems of the 2007 Nissan Sentra

The car ran fine, until one day the electrical system seemed to die (all the lights on the dash illuminated) and the car seemed to lock itself up. The power steering stopped functioning and made it hard to drive. After pulling over to restart it, it would not. At first I thought it may be the battery, but when I took it to the dealer they were unable to figure out the exact problem. It's been nearly a week since the problem started and no resolution yet. I chose to report this because when the car began to malfunction it became very unsafe to operate. Since I was in a fairly residential area it wasn't hard for me to pull over and get help, but had I been on a major roadway I could see this problem being potentially disastrous. I also feel that this problem needs to be more closely looked at since the technicians were not able to accurately identify the problem because it had yet to be seen until I brought my car in.