Was there a login prompt from Access or from Windows? If from Windows, then I would say that there is some kind of permission to access files or a problem with access to the network. If from Access, then I would say that something in SYSTEM.MDW that you are using is corrupt or has been reconfigured.
If the login prompt is from ODBC, it probably means that the credentials that are used to access the backend database (for your comments you mentioned in SQL Server) are either invalid or disabled. (Or it may be as simple as the database database / temporarily unavailable).
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