You haven't given us much to go on, but gazing into my crystal ball, I suspect you're trying to use Office Interop to manipulate Microsoft Office documents on the server, and Microsoft Office is not installed.
Read the following Microsoft knowledgebase article:
Microsoft does not currently recommend, and does not support, Automation of Microsoft Office applications from any unattended, non-interactive client application or component (including ASP, ASP.NET, DCOM, and NT Services), because Office may exhibit unstable behavior and/or deadlock when Office is run in this environment.
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