If you have hidden your Sign-In button (through Site Admin Options in the Toolbar), the button is not gone, it's just hidden.  In fact, it's right where you left it.  If you run your mouse slowly over the area where the sign-in link used to be, your mouse pointer will turn into a hand, and you can click in order to get your sign-in box.  If you just can't find it, contact our Support staff at support@spinsite.com.