Can you explain what are you trying here:
Convert.DBNull.Equals(TextBox11.Text);
Surely, it does not look like what you intend to.
BTW, a simple VS Debugger would had told you the error line and error in itself is quite self-explanatory.
I want to store the data(NULL) from textbox to data base.
Try:
cmd.Parameters.Add("@p", SqlDbType.Float).Value = System.DBNull.Value;