Instead of fixing it, you need to use a different method to power the facilities in the game. Here's how the story unfolds:
* The Broken Generator: You'll find the generator broken in the power plant in Goldenrod City.
* A New Solution: Professor Elm suggests using a special "Power Plant Key" to activate a hidden power source.
* Finding the Key: You'll need to complete a specific quest line to acquire the Power Plant Key. This involves helping the people of Goldenrod City with various tasks.
* Using the Key: Once you get the key, you can use it to power up the facilities.
So, instead of repairing the generator, you'll be using a new method to power up Goldenrod City.
Let me know if you have any other questions about Pokémon HeartGold!