Locking and unlocking a range of cells based on a fomula cell value change.

Put this in the sheet that has the manually changed cell so the code is triggered when you change that cell.
It will Lock\Unlock cells on "Sheet2".

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Range("A1"), Target) Is Nothing Then
        With Sheets("Sheet2")
            .Unprotect
            .Range("B30:C30").Locked = (.Range("A43").Value = 0)
            .Protect
        End With
    End If
End Sub