I had the same problem too and couldn't log in with http://www.mydomain/wp-admin/index.php or http://www.mydomain/wp-admin.
Here's how I fixed my problem without having to delete the plugin...
1. Open your ftp progam and go to the plugins folder (I think it's in the wp-content folder).
2. Look for the file "unavailable.php" (without the quotes. It shows as a PHP script)
3. Right click on the file.
4. Select "rename"
5. Rename the file (for example, unavailable2.php). Once file has been renamed you can exit your ftp program.
6. Go to your log-in page for WordPress.
7. Your log in page should now show up and you should be able to log into WP.