If you’re adding new plugins to your WordPress site, depending on the plugin type you can end up with your site being inaccessible if the plugin configuration is wrong. A good example is adding Google’s reCAPTCHA plugin. If you activate the plugin but the config is not correct, for example if your site id is not matching your domain name, AND if you’ve added the reCAPTCHA on your login form, you can end up in a position where you’re completely unable to log on to your site.
Luckily if you can ssh into your server, you can disable a plugin easily by just removing or renaming the plugin folder, for example in your wp-content/plugins folder.
More info in this article.