Carefully examine circuit board and all components, closely using good lighting and a magnifier.

Look for defects such as: Cracks, Burn Marks, Signs of Corrosion, Swollen Components, Unusual Looking Solder Joints ...

See pictures for example of Solder Joint Defect from this project.

Note that the defective solder joints were likely a factory flaw that eventually gave out or were caused by strain, due to too short of wiring as visible by connector angle.

See Step 3 for likely cause of strain on connector that resulted in fatigued solder joints. ( Ref: Exhibit - 1 )