Here's how you can restart Notification Center in OS X on your Mac. 1. Open the Activity Monitor (you'll find it in the Utilities folder). 2. Type the word "notification" without the quotes into the search box. 3. Click on the process that is listed. 4. Click on the red Quite Process button. 5. Notification Center will restart. This is a good way to fix Notification Center if it starts acting weird at some point.
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How to Restart Notification Center in OS X | OS X Daily