1. The plug that goes into the laptop is damaged
2. The power supply cable itself is damaged.
3.The internal power circuit board fails.
If the power supply internal circuit fails, it's not worth being fixed as a replacement power supply will be cheaper.
The only thing that is worth fixing is the plug at the end and/or replacing the cable from the supply to the laptop
If a particular power supply is not in stock, it can be ordered in, usually the next day.