In Word 2000/2002/2003:

• Click the "Help menu" and select "About Microsoft Office Word."
• Click "Disabled Items."
• Highlight any EndNote item(s) and click "Enable."
• Click Close.
• Click OK.

In Word 2007, click on the go to the Home button and select the Word Options link at the bottom. In Word 2010 click on File and select Options.

In the left hand column select the entry Add-ins, then at the bottom of the screen use the down-arrow to select the entry 'Disabled Items'. The following Screen shot is from Word 2007 but is similar in Word 2010.

Click on Go. You will get a window like this:

This example has no disabled items, but if you see anything like Endnote, or Citewyw or similar, then highlight them and click Enable. Restart Word. This may fix the problem.