Alernator?

My vehicle was down for 6 weeks, had not been run, so I put a new battery in and started the vehicle, but my alternator warning light came on and mt amp meter did not give any reading. A few days later I started the engine and all of a sudden, the amp meter needle went from 8 o’clock to 12 o’clock.
The dash lights went brighter than bright, the alternator warning light went out. All is well now!
What caused all of this to happen?