If you are getting the 'bummer' message, it's likely Etsy has suspended your messaging ability because they believe you were spamming. If you've been sending the same or similar messages a lot, that could have triggered it. If you really need to contact the buyer, you should have their email. Etsy is the only one who can give your messaging ability back and it can take weeks. The moral of that story is don't spam.
If you cannot respond because the username no longer exists, then it was a message from a scammer and Etsy has already removed them. Mark those messages as spam and do not engage.
