Record what you noticed
Write down the symptoms, when they began and whether anything changed beforehand. Exact observations and error messages are more useful than guesses about the cause.
Bring relevant accessories
Include items needed to reproduce the issue, such as the device charger. For an intermittent problem, a short recording may help explain what you observed.
Protect important files
Back up accessible files before service where possible. Never include account passwords in an enquiry; ask how any access needed for testing will be handled.
This article provides general information, not a diagnosis. Recommendations depend on the device and the specific symptoms.
Discuss a device issue