“I've added a header to my Word document, saved it, and closed it, but every time I reopen it, the header has disappeared. How can I keep it on my document?”
This will happen if you accidentally clear a particular checkbox in your Word Options/Preferences settings.
Your document suddenly looks like this:

When it should look like this:

The header and footer data are not lost. Your Word Preferences setting is simply suppressing them from displaying on the screen.
The resolution: Make sure your Preferences are set to “Show white space between pages in Print Layout View”
In Mac Word 2016 this preference is located at Word > Preferences > View

- In Windows PC Word 2016 this preference is located at File > Options > Display

Written by Richard V. Michaels, M.Ed.
Chief Product Architect, Great Circle Learning
Microsoft Word MVP