Next time that happens, try this. It's worked for me.
Find and re-name your Normal.dot file. This is often buried in your user profile. (ex. C:\Documents and Settings\[User Profile Name]\Application Data\Microsoft\Templates *). Re-name the file. You cannot rename the file if Word is running. It does not matter what you name the file: I usually add a single letter to the front of the file name: "onormal.dot." Then open Word. Word performs a check on startup to ensure it can find its normal.dot. If it cannot, it creates a new, blank default normal.dot. The file it leaves you when it exits is a brand new fresh template.
* Note: if you don't see the 'Application Data' folder, it means it is hidden. In Windows Explorer, go to the Tools menu, click on Folder Options, select the View tab, put a dot in front of 'Show Hidden Files and Folders' and hit Apply and OK.
As far as HOW it happened... here are some thoughts:
If you use any of the following features, your documents are likely to corrupt: Master Documents, Nested Tables, Versions, Fast Saves, Document Map, and saving to a floppy.
In addition, saving when resources are low can cause corruptions. If you notice Word start to slow down noticeably it's always best to quit and restart Word immediately; to close any other applications that are open; and to clear the clipboard, by selecting any character and copying it.
Other signs that you are low on resources: fonts suddenly not displaying properly; the wrong application icons appearing on your Desktop or in Windows Explorer (e.g. Word's icon appearing where Excel's should be). If you get these symptoms, restart Windows immediately.
A corrupt printer driver can corrupt memory; and if you then save, this can corrupt the document. Symptoms: Word often crashes when printing (cure: reinstall the driver).
A corrupt template will corrupt any new documents based on it. A corrupt Normal.dot template is especially bad -- it spreads its evil almost like a virus to almost every new document you create.
If you create list numbering using the Format + Bullets and Numbering dialog, this is likely to lead to a corruption arising eventually, especially if you also have the "Automatically update document styles" checkbox ticked on the Tools + Templates and Addins dialog (less of a problem with Heading numbering than other types).
A bad sector on your hard disk can corrupt a document saved to that sector. Make sure you run Scandisk regularly. Running Defrag regularly will also help reduce the chances of running low on resources. Encourage users to save to the network. Make regular backups.
Corruptions are usually, but not always, stored in Section Breaks. The final paragraph mark in a document contains a hidden Section Break, so in a single-section document, corruptions tend to be stored in the final paragraph mark.
Corruptions can also be stored in any paragraph mark in a document; or in an end-of-cell or end-of-row marker within a table; or in a bookmark. (Corrupt bookmarks are very rare in Word 97 and above, unless you have been using EndNote).
If you find that certain commands such as Edit>Find don't work within a certain table, that table is probably corrupt.
If you find text mysteriously disappearing and reappearing as you page down past a particular paragraph, that paragraph's paragraph mark is likely to be corrupt.
