[Note: I'm using Cmd- the same as Cmd+ as you don't actually type the hyphen or plus, and Cmd is the "command" keyboard key. I found the problem using Excel with Office 2011, but it may apply to Word too.)
Cmd-C, Cmd-V, Cmd-X and maybe others stopped working and the shortcuts weren't displayed in the menu after I was messing around trying to add Paste->Transpose to the tool bar. (In addition, selecting an area and hitting Cmd-C did not display the crawling-ants indication around the selection.)
Give this a try, and it may work. I had flakey results: go to menu View -> Toolbars -> Customize Toolbars and Menus...
In the window that appears the Toolbars and Menus "tab" is displayed (for me).
Check or un-check "Show ScreenTips for toolbar commands". (That setting is supposed to show what an icon is supposed to do if you hover the mouse-pointer over it.)
Once I UNchecked that box the shortcuts started working again (after "OK" dismissing the window) and the shortcut keys were displayed in the menus.
BTW, "Show shortcut keys in ScreenTips" is checked for me.
Now as I wrote this posting I found out that Cmd-C stopped working again. I went back in and checked "Show ScreenTips for toolbar commands" and Cmd-C started working again. And after more experimentation no matter if I check or un-check "Show ScreenTips for toolbar commands" I can't get Cmd-C to copy. (However the Edit menu does show that Cmd+C does "copy to scrapbook".)
So I exited Excel 2011 and went back in and all the shortcuts are again working and "Show ScreenTips for toolbar commands" is un-checked. I checked it and the shortcuts continue to work and hovering over an icon displays what function it's supposed to perform.
Therefore, the above may or may not work for you.
I still haven't found a way to make a transpose tool bar shortcut, although I read in another forum that Cmd-E is supposed to paste->transpose, but it doesn't for me. (You can get transpose if you use the Home tab's Paste ribbon pull-down but I'm trying to avoid mousing.) I might have to make a macro for it.