Hi Bee!
Most of the time caused by 2 things.
1: it could be the PSU. Replacing this first is advised and very easy to do. It would be a great test as well.
Guide: [guide|103362]
2: It’s the APU. This will be very difficult to repair. You can either reflow it (most easy method) but most of the time temporary. Or reball it, this requires some serious soldering skills and devices to get it done.
If you need any guides for reflowing and reballing, please let me know!