Stop using "magic numbers" like "70130" in your code - they make it very hard to maintain, and are prone to problems like this.
And in this case, your code is pretty bad - you set the textbox to every single line of the file, and always overwrite the last one.
Your code could easily be replaced with:
string[] SinhalaWords = File.ReadAllLines(Server.MapPath("~\\Resarch\\new.txt"));
TextBox1.Text = SinhalaWords[SinhalaWords.Length - 1];
Which wouldn't have the problem and would work a whole lot faster!