The addon's name is KillCounter?
Logout first!!!
Then have a look at the SavedVariables folder at c:\users\your windows username\documents\Elder Scrolls Online\live (or liveeu)
Inside this folder there could be a file named KillCounter.lua storing the settings for the addon.
Make a backup of this fileso you are able to restore it later if something breaks the addon.
Open it with a text editor then and search for the charactername where the window is not shown anymore.
It should look like this
Lua Code:
["Your Character Name"] =
{
...
},
...
Between the { and } there are saved the settings for your character.
Look for something like x or y coordinates or top and left positions and reset them to 100 so the window will be shown at the top left corner with 100 pixels margin to the edges.
Save the file.
Login again and try if it works.