The question is "has this work been published or not". If it has then you probably can't publish it elsewhere. But there is a question here about the situation.
I suggest two things. First, contact the original conference chair and explain in detail what has gone on and get their opinion about whether it is published or simply presented. The editor of the associated journal would be another (perhaps additional) source. If they agree that it hasn't been published then you are free, assuming that you haven't signed away any rights.
Second, I suggest that if/when you submit to another conference/journal, that you again explain, in detail, what has gone on. If you have the backup of the first conference chair, I suspect that things will be fine, otherwise questions will remain. It will be their decision, of course, but it will be made with full knowledge and perhaps some useful feedback.
If the basic structure of the conference system in your field is just to "present" new work early, prior to publication, then it is likely that people will agree that it hasn't been published and can be. But if the tradition is (as in CS) that conferences are the main "publishing" venue, then it will be harder.
Another aspect to consider is what, exactly, was published in the first conference. If it was only an abstract, then likely the "work" hasn't been published.
Yet another avenue to explore is whether the first journal's publisher will, themselves, accept your work for publication. If that is the case then almost all difficulties will disappear.
But the big idea in this answer is that you may not be able to make the decision on your own and the advice of other parties needs to be considered, and, that, with full knowledge.