I plugged in my Arduino to my MacBook when my Arduino already had a 12V source going into it and now my MacBook won’t turn on and is producing a humming sound. Is my Arduino the cause of this? Is there anything I can do?
It sounds like you back fed 12 volts to the mac USB port via the Arduino and killed the mac.
Most reputable computers would handle that by simply shutting down the computer or USB ports until the fault was cleared and a restart was initiated.
Remove the Arduino and its 12 volt connection from the mac along with any other USB items.
Restart the mac.
If you are lucky it will restart.
If not you killed it and it may need one of those expensive mac repairs.
An expensive lesson in checking your voltage connections or a frightening experience not to do it again without checking,