want to check if USer exists

hi,

I have a data table called  Users-
Users
Name -  nvarchar
UserId   GUID
DateJoined nvarchar

On my aspx page I have a photo gallery and below is a   button  called FEEDBACK.
I want to wire up an event so this happens when the User cLicks the feedback button

1, If the user is logged in, and the the User exists in the "User Table"  UserId for example
it opens the modal window from code behind  which is a ModalPopupExtender1

2, It the user is not logged in redirects them to another page

thank you

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