Excel 97 Validation Radio Button

Sounds like the sheet wasn't protected, and someine accidentally deleted the button.
If you want to avoid this in the future let someone protect the sheet where the buttons are in and let data/information be stored in another sheet.
And another thing, while a workbook is shared, you cannot make changes to the restrictions or messages by using the Data Validation dialog box. When you enter data in a shared workbook, restrictions you specified before sharing the workbook remain in effect, and messages you specified before sharing the workbook continue to be displayed.

Here's how to Prevent changes to data restrictions and messages

-Unlock the cells for which you have set up data restrictions and messages and in which you want data to be entered. To unlock the cells, select each cell, click Cells on the Format menu, click Protection, and then clear the Locked check box.
-On the Tools menu, point to Protection, and then click Protect Sheet.
-Make sure the Contents check box isselected.
After you complete these steps, the Validation command on the Data menu becomes unavailable.

I agree that the button was probably deleted by your user. Another way to keep the button in existence while having it seperate is to create a form and place your controls on the form.

I did this once on a workbook that we had users entering data on a daily basis and doing some basic calculation. I also put in buttons that would do the calculations for them and place the value in box x.y (active cell, or column x of same row of active cell)

Once you have created the form, you can then have it available and yet the design element will be seperate and safe.

hope this helps.