If one has uploaded plugin and activated it then it may be possible that due to this your wordpress blog may be blocked. If this case arise then one can over come from it via below mentioned process:
1. Using an FTP service, navigate to your WordPress plugin directory /wp-content/plugins
2. Locate and rename/delete the recently activated plugin.
3. If one has made any plugin code modifications to template files, undo those changes.
4. Once finished, try to log in to your WordPress admin panel. It should be fixed.