i don't know what's wrong with it can you analyze if possible - system

Question

I have based my codes from my professors. I typed the right username and password but it keeps on saying invalid user. The name of the table is also correct. I don't know why it keeps on saying it's an invalid user kindly help me please.
If Len(Trim(txtUsername.Text)) = 0 Then
MessageBox.Show("Please enter user name", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
txtUsername.Focus()
Exit Sub
End If
If Len(Trim(txtPassword.Text)) = 0 Then
MessageBox.Show("Please enter password", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
txtPassword.Focus()
Exit Sub
End If
Dim query As String = "SELECT * FROM Users"
Dim dt As New DataTable
ExecuteQuery(query)
Dim ctr As Integer
If dt.Rows.Count > 0 Then
If (dt.Rows(ctr)("Username")) = txtUsername.Text Then
If (dt.Rows(ctr)("Password")) = txtPassword.Text Then
frmMainMenu.Show()
Me.Hide()
End If
Else
MsgBox("Incorrect password")
txtPassword.Text = ""
txtPassword.Focus()
End If
Else
MsgBox("Invalid user")
txtPassword.Text = ""
txtUsername.Text = ""
txtUsername.Focus()
End If

Answer 1

You are never initializing the dt DataTable. So a new DataTable will have 0 rows, and you will fall into the Else of If dt.Rows.Count > 0. I presume you had intended to have dt be the DataTable that results from the ExecuteQuery statement, but as you are neither passing dt in nor assigning a result from ExecuteQuery to the dt variable, it is still blank.
Not knowing what ExecuteQuery does, I would imagine it should be either
ExecuteQuery(query, dt)
or
dt = ExecuteQuery(query)
After which point you might fall into the intended username/password comparison block, if there are actually rows in the Users table.
However your matching still will not work unless the matching record is the first in the DataTable, as you never actually iterate/increment ctr.

Answer 2

this is my codes for ExecuteQuery()
Dim connectionString As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\nikko\Documents\Visual Studio 2010\Projects\PayrollSystemThesis\PayrollSystemThesis\bin\Debug\Thesis.accdb"
Public Function ExecuteQuery(ByVal query As String) As DataTable
Try
Dim dbCon As New OleDbConnection(connectionString)
Dim dbDA As New OleDbDataAdapter(query, dbCon)
Dim dbCB As New OleDbCommandBuilder(dbDA)
Dim dbDT As New DataTable
dbDA.Fill(dbDT)
Return dbDT
Catch ex As Exception
MessageBox.Show(ex.Message)
Return Nothing
End Try
End Function
it reads sql statements.