Most likely this is due to a bug in the window position logic to try to remember where you previously closed the editor from so the next time it's loaded it appears in the same screen position and size. In some situations, I've seen this get mangled and produce a value that is off the screen. I've got a fix for this but it hasn't made it's way up to DevCentral for auto-updates.
In the shorter term, if you close the editor, load this registry file, and restart, everything should be peachy...
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\F5 Networks\iRuler]
"WindowPosition"="100,100,1000,1000"
You can also manually edit the WindowPosition value with the Registry Editor (regedt32.exe).
-Joe