It sounds like some sort of social engineering or phishing.
If the site isn't one that you actually have a login for or visit, that is highly suspect. If they are giving instructions (in said unsolicited email) to click a link in the email and then enter your details then it is most likely to be phishing trying to steal your user name and password.
It is possible that your email address has been harvested - possibly from someone's email address book, whose system is infected.
You should never respond to unsolicited emails, never click links or open attachments in unsolicited email.
