Dunno - had you modified your default.rui popup and not saved your workspace under a different name? Not a good idea… You might find your modified toolbar in the same folder as the normal .rui -
it will have a rui_bak extension on it, check the date/time it should be something close. Try renaming that to something.rui and open it. You might find your toolbar if you haven’t opened/closed Rhino a couple of times since the problem happened. Otherwise, you’re probably out of luck… Always save modified workspaces under another name.