Check the value then procede with sql.
If Not String.IsNullOrEmpty(Combobox1.Text) Then
Dim sql As String = "INSERT INTO Daisy_March2014 " &
"(ChargeCode,Peak,OffPeak,Weekend,Setup,MinimumCharge)" &
"VALUES ('" & ComboBox1.Text & "','" & ComboBox2.Text & "','" & ComboBox3.Text &
"','" & ComboBox4.Text & "','" & ComboBox5.Text & "','" & ComboBox6.Text & "')"
End If
You should also look into Parameterized queries for protection against sql injection.