New users, honestly, I wouldn’t reach out to. I just consider them lost leads unless they’re in the newsletter list and then I would announce it via that

writen by WBE User
If the users complained or you know they must have noticed: send an email with apologies, explanation and that you fixed it. If somebody made it public, also publish the email’s text somewhere. Else: just move on

writen by Benedikt
Since they are only 3 I would reach out to them personally (email, or twitter). It’s also a great excuse to ask for feedback

writen by Tiago Ferreira