Hi Subbarao,
1. If Bex Query is greyed out, then it is not been enabled for external access.
2. If Bex Query failed to retrieve cube for the connection, then the Cube might be removed or relocated (or) Connection fail
Try this workaround, this connection will not be affected if you relocate the cube in BW side and also you can use any cube / data mart to access data from BW.
Create a new secured OLAP Connection with SAP BICS Client under the SAP Netweaver BI 7.X option by providing credentials. Select "Do not specify a cube in the connection" and click finish then publish it to the BO Repository.
Open the WEBI Report, select the Bex Query as Data source which you want to access, now you can able to retrieve the data from the desired BW Cube via BEX Query.
Check the User Rights and Role imported into BOE Repository.
--Raji. S