Any ideas? I have automatic backup from my host....just wondering what file I would need to rollback.
Long shots.
1.
Investigate the database of a previous backup, get the affected user's email addresses and invite them to re-register. If there's a "ton" of them, like you say, that is going to take some time to compile.
2.
I'd like to say try and investigate the database of a previous backup, adding these unconfirmed users back into the xf_user table but I'm pretty sure that's not that easy.
I spoke with Jake once a while ago about adding users back , or even adding users into a previously deleted userid # - there could be later trouble with any associated tables.
3.
Depending on when your host did the last backup (*some* hosts do this on a six-hourly basis) - you stand to lose any interaction that your members have made with the board between then and when you do the "roll back". A figure this roll back would in this case have to be a database recovery job.
Depending on just how busy your board is, the severity of a database recovery will vary.
So, unless anyone else can think of a better way in the meantime...
Contact host, quickly. Ask (so you get a clue of how far back you're going) when the last backup was completed.
Ask for a database and file structure restore to their last backup - they should help you through this, or do it for you if they're any good.
After the work has been completed, I'd put up an apology notice.