Is that store available at admin site?
If available you can download code of that theme and take backup as:
Nick here from Shopify.
I would need to reach out to you directly to see why your store is unavailable right now. It could be a multiple of different things and without looking into it, it's difficult to know exactly what might be happening. Is the best email to reach you on the one you signed up to the community with? If it isn't could you let me know what is, please?
Regarding backing up your store/ theme, @oscprofessional hit the nail on the head with their screenshot. To follow up on the screenshot, Shopify has a helpful guide which you also might find useful here about it.
All the best, Nick