Sorry to hear that, it sounds like you will need to restore your forum from backup
Currently we are running a hacked up version of vBulletin and am very interested in seeing if we can achieve this functionality with NodeBB.
In order of complexity - can this be done (with or without intensive code modifications).
Shared code base ... currently we can point all subdomains to a single code instance and depending on the subdomain change some variables in the init functions and we're generally all set. In its simplest form - point to a different redis / mongodb database / url (i.e.. change the config.json variables). This would allow us to at the very minimum run multiple domains from one code base.
Shared users... Going beyond the shared codebase but different redis/mongodb setup, would it be possible with a few modifications to share user authentication between these different forums. In vBulletin for example, we've done this in the past by modifying the code to point user related queries to the user / groups tables in the main forum database. Private messages, notifications etc stay specific to the sub domains in this setup as they are driven from different forums.
Share forums... With shared users enabled, we also have modifications which allow us to share forums between domains (for example "site related / trouble shooting / global news" type categories). This requires us to operate all domains from a single database and on init filter out the forums which are visible depending on the forums visited. Was actually quite simple to do with vBulletin as we could easily plugin at a "before_init" type event and setup the forum category variables depending on our config.
I ask this as we're reaching the end of life on our PHP setup and am keen to look at a platform which will take us forward over the next few years.
For 1 I would be inclined to use clustering then reverse proxy the subdomains to categories. Rather than spooling individual instances for each. I've done this at my own site.
dnb.35hz.co.uk will take you to the electronic category.
You could then make a few changes to how your homepage/main website looks and do it that way.
As an additional. Anything you're not sure exists, drop an email to sales [at]nodebb.org and they may be able to create what you need.