if you opened the rc file as text, it can lock you out. Its easy to accidentally open it in a tab, or even another editor. Ive had it bug up as well, where nothing has it open but visual thinks otherwise ...
If the IDE has the resource definition header file and/or the resource script file open as viewable text file(s) VS whinges the resource design tools can't be opened. I know that from experience, having done this numerous times.
Normally I do any resource editing as text in the header and resource script files, when using the resource design tools VS can add a lot of what I consider extraneous verbiage to the files.
VS can open user edited header and resource script file(s) in the resource design allowing the designer to visual view the resources without adding the bloat. Tweak or add resources, though, and IDE design text is added back to the files.