Why is my iPhone not syncing to my computer?

For my than a month, my iPhone has not been syncing to my computer. When I plug it in to the cord, it will charge but iTunes won't recognize my iPhone at all. Therefore I can't put any new music or videos on my phone. This is not the first time this happened, but it is the longest time its happened. Also I've tried my brothers iPhone and it still does not pick it up. How do I get my computer and iTunes to recognize my iPhone?

You may refer to this tutorial, it's about how to fix iTunes sync problem with iPhone.

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