PowerPivot Table Update Fails

Hi,

Could some one please shed some light on how to resolve the following error in PowerPivot?

OLE DB or ODBC error: Login failed for user 'reporting'.; 28000.
A connection could not be made to the data source with the DataSourceID of '88c88e17-3186-4c5b-ba82-9e986e693545', Name of '____________________'.
An error occurred while processing table '______'.
The current operation was cancelled because another operation in the transaction failed.

This error occured when I tried to "Edit Table Properties" in PowerPivot data model to update the data set by adding new column names from the SQL source. It had worked fine in the past but not sure what changed now. Any thoughts?