How exactly do you lose an online account?
The wrongs of Paypal have been mentioned, but most account blocking happens when a user violates Steam's TOS. Being an abusive jerk online while logged in and playing games. Or cheating.
Now, I think that's bullshit. You still paid for those games. I think you should own them. Completely. At most they should block you from their servers and their online play.
But with Steam you can easily download any title you own as many times as you like. Steam has its own problems, but this they got right. You can even back them up quite easily. (Man, that used to be a broken tool, but even it's good now.)
A TOS ban means your account is no longer usable.
The database could get corrupt or the data center burnt down and if the service provider hasn't a good backup strategy your account could be gone. The same goes for unauthorized access to the database or your account, it can be removed without your say so. Or the service provider is shut down by the government for whatever reason. Or worse, your country decides that the service provider may not operate in your country and that all accounts of people from your country should be removed and they comply. Or some governmental agency takes all the servers of your service provider and won't give them back (in one piece).