Login page code in Silverlight 5 and LINQ-TO-SQL

Question

Dear All,

Please help me out. I need Silverlight 5 code as soon as possible. My requirements are:
I have a table as follows:

ID(PK), int	Username	Password	Role
1	A	abcd	IT
2	B	efgh	User

:

I have a Silverlight Application and I have added Silverlight-Enabled WCF Service and LINQ-TO-SQL Class.
I want that:s
When the user enters the username and password in the textboxes, the system should check whether the user is present in the database or not.
If yes, the system should return the role of the user.
Eg-if the username and password entered are A and abcd respectively, I want IT on a textbox.

Please do help me out as I am not able to use the Single() method of LINQ-TO-SQL Class.

Thanks in advance.....