[Stiglr]

If you haven't saved the project since you merged things, you could use Ctrl+Z to "undo" steps until you get back to where the objects were before you merged them. But, if you've taken time to make a post, I'm sure it's at least a day later and you've saved or powered off your computer since then. Sadly, you'll now have to do it "the hard way"... good news is, it might not really be that hard.

In Surface Select mode, select the surfaces of discrete parts you want to separate. You'll likely need to make use of wireframe mode to be sure of what you're selecting. With each grouping selected, use Surface/Cut Away Object to separate the selected surfaces from the greater object. Then, re-name the new surfaces as their own object in Object Select Mode. Before selecting the next group of surfaces, Lock or Hide the new(er) objects to avoid confusion.

Rinse and repeat until everything is hidden and composed of new objects.