It means your work tree and the remote tree you just added do not have any commit in common. That means Git cannot know what to do with them.
So start examining the log of both repositories using bellow command.
vimdiff =(git log master) =(git log remotes/origin/master)
If it is confirmed that the trees do not have commit in common, then check each commit one by one and try to match each commit in both the location.