OK, I have some things myself I have to deal with right now, so the next few days I'll be travelling and won't be able to respond immediately. I'll do my best to help you though.
I'm assuming your server is running on a LAMP (Linux, Apache, MySQL, PHP) setup? If not, you might need to change some things in my instructions to fit your setup.
If my instructions below don't make any sense to you, get a friend who knows a bit about how to use Linux to read them, or show them to a technical support person at your webhost.
Firstly some questions:
1. Do you have a backup of your database from before you installed the plugin? (If you don't you're going to need someone else to help you, preferably the plugin author because he might know what he's broken.)
2. Do you have access to/know how to access PHPMyAdmin on your server? (If you don't know ask your webhost for help with this)
3. Do you have FTP access to the server?
If you answered yes to all those questions, then one process to fix this is:
1. Connect to your server by FTP or SSH.
2. Open your wp-config.php file to find out what the name of your WordPress database is.
3. Log in to PHPMyAdmin.
4. Backup your current WordPress database to a new file and then drop that database (that means delete it - don't drop the database user - we still need them).
5. Import your old backup file from before you installed the plugin to recreate the database (if it says the file is too big, see below).
6. Edit wp-config.php to change the database prefix back to what it was before - the default is "wp_".
7. Save the file and upload it back to the server, then try logging in again.
8. You might need to restart some services if you can?
If things aren't working, try restarting Apache by typing:
service httpd restart
if that doesn't work... try
By default PHPMyAdmin doesn't allow very large files to be uploaded through the web interface. If your backup is too big you may need to edit /etc/php5/apache2/php.ini on your server. Change the following values to:
memory_limit = 64M
post_max_size = 64M
upload_max_filesize = 64M
Then save the file and restart Apache.
(this assumes that 64MB is enough for you to upload the file)
I hope this helps.