After looking into bitcoins (on and off) for over 2 years, and finally started to accept bitcoins, I've added some Bitcoin news sites to my feed.

As a result, I keep hearing stories about people finding their old blockchain account is full of bitcoins, but in many cases it's a local wallet and the hard drive has been formatted 3 times since 2009.
What actually happens in those cases? Is there a way to assess the amount of those so-called inactive bitcoins?

I suppose that because of the security features of bitcoin they're probably inactive forever.
Could that be the case? Has anyone measured the amount of bitcoins that are inactive for over 2 years? Could that be a potential problem?