For starters, don't do it like that! 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. Always use Parameterized 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 concatenating strings on a login page? That's just plain asking for your DB to be deleted as I don't even have to sign up to do it!
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.
This is the one you noticed, and it's the trivial one: no records match your query, so there are no rows returned. No rows returned, means you get an error saying "you can't use a row that doesn't exist" which is what "There is no row at position 0" means.
But fix the first two throughout your app or your DB is toast (and you are wide open to massive fines for GDPR failings).