I know this is the boring answer but I’d just buy the modern stuff and try to complete master sets.
That is of course m where most of us started. And after the heartbreak of losing all my other cards, I think it’d be a decent way to still have enjoyment in the hobby.
Assuming I didn’t drop the hobby I’d purchase the holo sets of Base/Jungle/Fossil in English unlimited then in Japanese. I like to collect unlimited wotc sets and don’t have a personal preference for a certain pokemon or artwork. I generally prefer to purchase the most expensive pieces first.
Well, only looking to buy my first graded card now I guess it would be Shining Gyarados.
However, if we’re talking about collection as a whole, and my binders wen’t poof too, then I’d probably say Fossil Lapras, or Black Star Eevee promo 
I started with Hidden Fates when I started again last year. But then realized nostalgia beat everything so went back to 1st edition base set and WOTC cards.
I really don’t like to think about this scenario because it’s entirely possible my collection could be stolen or something since it’s actually worth something now lol.
Like everyone else said, if it happened, it’d be really hard to get back into it again. There’s probably 3 main things I’d be on the look out to buy.
Shining legends packs as that was the set that got me back into collecting again and it want to try and complete it again, buy crystal celebi and birthday pikachu on eBay, my two favorite cards, and scour Facebook marketplace or craigslist to buy big old collections to start binder sets again.