I suppose the first question is to make sure you're playing the latest patch. By this point, considering where you'd have to get the game from, that's probably true, but it can't hurt to make sure.
If not, you may have simply run into the game randomly deciding to bug out. Do you have a save from before you got the letter? You may have to reload from there and try again. If that doesn't work, it may be that character's game itself that's bugged, which is sadly plausible when it comes to Daggerfall...
(...Also, I have a strange wish to go play Pharaoh again for some reason.
)