You can't unpublishing default currency, and it is an issue from our end too we will fix this and add a check so no one can disable default currency. Thanks for notifying this issue, we will release this fix in our next release.
As of now if you facing any issue by this let us know, we will fix for you.
It wasn't default currency though, I had already changed that to GBP UK Sterling before hand?
Anyway did a reset and that at least got the system working again, but now I have no sample data etc all gone.