Try with the following code.
Control myForm = Page.FindControl("Form1");
foreach (Control ctl in myForm.Controls)
if(ctl.GetType().ToString().Equals("System.Web.UI.WebControls.TextBox"))
((TextBox)ctl).Text = "";
Try with the following code.
Control myForm = Page.FindControl("Form1");
foreach (Control ctl in myForm.Controls)
if(ctl.GetType().ToString().Equals("System.Web.UI.WebControls.TextBox"))
((TextBox)ctl).Text = "";