In 2010, you came across a site that was giving away bitcoins. The site said it was free money so you signed up. You were logging in to figure out how it was money but couldn't. The last time you logged in you had more than 100 bitcoins. Later you forgot about the site until 2017 when a ponzi was advertising Bitcoin. Bitcoin? The name rang a bell and you stated hearing stories. You remembered you must have many bitcoins somewhere but you can't even remember the name of the site. How would you recover your BTC?
The scenario that you are presenting is one in which is really difficult to recover your bitcoins, websites appear and disappear from the Internet all the time and if you did not took the time to create your own wallet and to store your private keys in a secure way then it is quite unlikely that you would be able to recover those coins after forgetting about them for seven years, for that kind of money you should try your chances but the odds are not very high.
But even if the scenario is not real it is important because it reminds ourselves that we need to take complete control of our coins because if we don't then something like that could happen and you could lose a fortune that you deserve.