What exactly happens to an NDA if the company goes under or is bought out?

You did put in clauses which indicate post-termination time-expiries? Legally there may be some jurisdictional issues but you didn't state what the NDA covered, so some companies expressly write in return of written documents or enjoiners on future ex-exployees. If you can't police the existing NDA, what hope to enforce a future scenario ... which just shows you shouldn't write down anything you can't exercise power over (though lawyers might disagree) either directly or through judicial/administrative means. If there was a law on mind-control, it would be Orwellian 1984 already.