Characters need to want something. If they don't, it's impossible to think up a plot, because goals drive the plot. So what do your characters want? If you can't answer that yet, try...
Do you have any kind of premise at all? A genre, and type of story, something to guide you? What are common plots in that genre? Common plots using that premise? (I suspect this is your first novel? If so, don't worry too much about being original and just work on the skills to plot out a novel and write one. Once you do that, originality will come. If it's not your first novel and you're just stuck on this one idea, then use other plots as a jumping off point to get you thinking)
You say you have a setting already, so maybe look closer there.
What are some bad things that happen in this setting?
What's the worst thing that can happen? Or ever did happen?
Is there anything that can still happening?
How can you connects any of these to your characters in any way?
Is there anything that your characters have to stop or prevent?
(These can be large scale issues or personal one-person issues, though a combo of both would likely give you the best plot)
Now, look at your genre/premise. Do any of those bad things lend themselves to they type of story you want to write? A love story if it's a romance, a terrorist attack if it's a thriller, a quest if it's a fantasy, that kinda thing.
Next, look at those characters with the above ideas in mind.
What are their problems?
What has gone wrong in their lives?
What do they want most out of life?
What do they want most at this moment in time?
What are their biggest fears?
What is their biggest flaw?
Who are their enemies?Why?
Now how might you take your premise, the problems the setting creates, your characters personal desires and fears, and turn that into a plot?
What do they want, that will be made harder by the problems of the setting, and made risky and dangerous by both the setting and their personal fears? What are they willing to risk all to do?
That should give you a plot, even if it's just a basic log line idea you can work from.