What other recent changes have been made? Is the site SSL or not? ZC version? Have you tried to check for a bad table in the phpMyAdmin?

In past similar situations, we have also found that an SSL change or 301 can block access. Is your admin expecting a certain URL and an added .htaccess setting is keeping you out?

e.g, if your site is set to redirect to http://www.YOURSITE.com but your SSL is for https://YOURSITE.com, you might be blocked if you use the wrong URL combination.