If you opened up Steam and saw that your game library is practically gone (or massively increased), it’s probably not your library. The PC gaming marketplace has been shown to have a massive security issue that seems to swap user info with that of other players seemingly at random. In some cases, just clicking anything may have you end up in multiple accounts each time.
If it’s possible, you should get to your account and change your card info. While you may be confused and not willing to screw someone over, that doesn’t mean the person with your account will feel the same way. We’ll keep you updated on Steam’s security issues as Valve gives a statement and offers a solution. Some people have been saying to try to unlink your account from Paypal, but that may not be such a great idea after all: