When i open my World Editor its black to, but its just because its a Quick Start thing i guess, when i open another map or create a new, everything works fine with me =)
I think you start the "Quick Start" map where everything is black, even unit models and open a Spell Map, then try to put it to the map that was black.
If not, please tell a little more about the proplem =)
There cannot run 2 Warcraft III World Editors at a time, thats why it says "World Editor is already running".
If you edit a Variable in a spell map or by a mistake change a name in a spell, it will ask you next time you open this map again "Save changes to (Map)", then you say yes for keeping the changes, and no if you dont want to keep the changes you made.
You can also look on the Windows Joblist (If you are using Windows) and find "worldedit.exe" and click "Close Progres" if there is bugs in opening maps and then you can try again.
I dont know if any of this helped, 70% i was bored, but still you need to explain more and come with details to get fast response.
Edit: Manually is that you can find the Warcraft folder in the "C" drive and open a map from there i guess xD