Your issue is that most likely the control being searched for is private or simply non-public. In the GetField call you should add Binding Flags to allow searching for the private / protected fields, if that's what you really want to do.
FieldInfo info = this.GetType().GetField("txtBoxName", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
if (null != info) {
object val = info.GetValue(this);
}