if you're talking about some sort of error message, there are asp.net validator controls that you can use, and one of them (the Summary Validator) can be displayed as a messagebox.
But other than that, there's no way the server side code can trigger something on the client.
Showing a msgbox is a client side event. There is no way WebServer A over in New York can trigger a message box on Client Machine B in San Diego or wherever. The only way is with javacript or vbscript, which will display the msgbox upon some event or action happening.
ie: the asp.net validator controls. When the page is rendered, all those controls do is write javascript to the page.
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