If by "label" you mean System.WebUI.WebControls.Label, then its <BR />
as in:

   this.label1.Text = "Label <BR /> Text <BR /> in C#";

On the other hand, if you mean System.Windows.Forms.Label, then its
the escape code, \n, as follows:

   this.label1.Text = "Label \n Text \n in C#";

When using the WinForms Label class for multi-line output, remember to
set its .Size property to have a height sufficient to contain all lines.

Derek Harmon