Assuming the file is in the root directory of your website - which is a poor idea - you need to prefix the filename with a "~" - if you don't it's assumed to be in the same folder as the page ASPX file:
string oldFileTxt = Server.MapPath(@"~/txtp1.Text");
A better idea is to create a folder specifically for files and use that:
string oldFileTxt = Server.MapPath(@"~/Uploads/txtp1.Text");