Relax, the file is probably just on a different path for your computer. What you need to do is find out where the file actually is, and then update the link so that Word looks in the right place. Proceed as follows:
In the Word document, choose Edit » Links to display the Links dialog box (see Figure 9-2). Click the link that's not working (the Update column will probably say "N/A" to indicate that the link is not available), and then check the "Source file" readout in the "Source information for selected link" box. If the readout is too short for you to make out the file's path and name, click the Change Source button and use the "Look in" drop-down list to see the full path.
Figure 9-2. If Word tells you that it can't find the linked file, you may need to use the Links dialog box to edit the link to match the file's new location.
Open a Windows Explorer window to the folder that's supposed to contain the file and try to locate it. Most likely, either the folder is on a different path for your computer, or someone has moved or renamed the file. If you're out of luck, someone has deleted the file.
Once you know where the file is, click the Change Source button and use the Change Source dialog box to tell Word where the file is.
In the Links dialog box, click the Update Now button to update the link.
In the Links dialog box, you can also break a link so that it no longer works. It's a good idea to break a link before sending a document to someone for whom the link will not work. Update the link before breaking it, and the document will contain the latest data available.