Nothing comes on when I turn the key

Start by charging the battery for an hour on the lowest setting. At four years old much of the battery's capacity is gone. A new one should be able to start the engine after sitting for up to three weeks.

Dandy. It should have held a charge longer than a week. You might want to consider having it load-tested at a battery store. Since GM started using their new design generators in 1987, repeat failures have been very common. The biggest thing you can do to avoid problems is to replace the battery. Two years old is kind of pushing it but by four years old they lose their ability to absorb and dampen the huge voltage spikes these generators produce. I haven't heard so much about problems on newer vehicles but that was a real common problem in the '90s. Replacing your battery now might avoid a generator failure in the near future.