Username, Password and UserType Validation
Here are some observations:


you first iterate all rows in Credentials and then select the Password yet again - why not just select the pasword?
seems like your password is plain-text - BAD idea - hash your password together with a  salt please
don't concat strings to get SELECT queries - use parameters instead
you are using C#/ASP.NET so why don't you use the provided login mechanisms?
you return a string "A" if the login succeeds and the error message if not - this is IMO bad design - return a simple struct/class with more information instead