wp-login not working!

Apparently without reason, the admin login function of a site I administer has stopped working and I can’t access its WP Admin. The site has only one User. When I go to http://inspiredwordnyc.com/wp-login.php or http://inspiredwordnyc.com/wp-admin and enter the credentials…nothing happens. No error, no rejection, no redirect. Just nothing. When I click “Forgot Password” and enter the user’s email address, it is rejected as invalid (the address has been double-checked on the database).

Here are some things I’ve tried based on earlier threads:

1) I tried resetting the plugins. No change.
2) I tried switching to a backed-up database that was definitely working when it was archived. No change.
3) I tried changing the wp-options fields in the database under “Site URL” and “Home” to variations of the same, such as adding the www prefix or subdirectory slash. No change.
4) I tried changing the wp-options fields in the database under “Site URL” and “Home” to the url of the development site on another server. This successfully logged me in to the development site.
5) I checked the site’s .htaccess file. It’s unchanged from the standard WordPress redirect.

I’m basically out of ideas and at a loss as to what to tell my client. Does anybody know what I can try next?