I think doing the regex one letter at a time and trying to work out the [c-z] stuff is going to be hard, and you'll have to do it 26 times!
I'd do something like this - go through each letter of the original word, and if the letter is one of the guesses, then output that letter, otherwise "_", and build up a string.
StringBuilder sb = new StringBuilder(); // start with empty string
for ( int i = 0; i < word.length; i++ ) // for each letter in the word
{
letter = word.SubString( i, 1 );
if ( guessed.Contains( letter )
sb.Append(letter); // add the actual letter (already guessed)
else
sb.Append("_"); // add a _ since they haven't guessed this one yet
}
newword = sb.ToString();