Change your code to this .. It will work
string txtfilesentence=string.Empty;
string word=string.Empty;
txtfilesentence = txtshowfile.Text.Trim();
word = txtwordmatch.Text.Trim();
if (txtfilesentence.Contains(word))
{
lblfound.Text = "word matched";
}
else
{
lblnotfound.Text = "word not matched";
}
Edited