Back up what matters
If the device still works, make a current backup of important files to a separate destination. Verify a few files can be opened.
Write down the symptoms
Note when the issue started, the exact error message and whether it happens every time. Photos of an error can help.
Bring the right accessories
Bring the charger or power cable. Include an accessory if connecting it triggers the fault.
Keep credentials private
Do not put passwords in a contact form or email. Discuss any required access during your appointment.