For starters, don't do it like that! So many things wrong here...
1) Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?
And doing it on the login page for a website? Where anyone, anywhere can do what they want? That's almost criminally silly.
2) Never store passwords in clear text - it is a major security risk. There is some information on how to do it here: Password Storage: How to do it.
] Combine that with teh SQL Injection, and I can log in as who I want, when I want, without even knowinhg a single password...
3) Get your SQL right.
new SqlCommand("SELECT count([Role]) Users WHERE ([Role] = 'Admin' ");
Without a 'FROM' before 'Users' that will cause an SQL exception.
Fix these, in that order as a matter of priority throughout your whole app: or you will lose your database - heck you best mate will try it to see what happens and just to see the look on your face...