You need to store the current word and letter position so they can be used in your click event
private string currentWord;
private int currentLetter;
private void btnStartGame_Click(object sender, EventArgs e)
{
MessageBox.Show("Game has started");
btnCheckWord.Enabled = true;
txtGuessWord.Enabled = true;
lblstart.Text = "Guess the word!";
this.currentWord = Word[random.Next(Word.Length - 1)];
this.currentLetter = 0;
char letter = this.currentWord[currentLetter];
lblSecretWord.Text = letter.ToString();
}
private void btnCheckWord_Click(object sender, EventArgs e)
{
if (txtGuessWord.Text == currentWord)
{
MessageBox.Show("Word is Correct");
}
else
{
MessageBox.Show("Word is incorrect");
this.currentLetter++;
char letter = this.currentWord[this.currentLetter];
lblSecretWord.Text += letter.ToString();
}
}