Wordpress - Activate a plugin through PHPMyAdmin or FTP?

I fixed this by going through PHPMyAdmin to the table "Options" and then the row active_plugins.

I had the following stored there (formatted for readability):

a:10:{
    i:0;s:49:"1and1-wordpress-wizard/1and1-wordpress-wizard.php";
    i:1;s:29:"acf-repeater/acf-repeater.php";
    i:2;s:30:"advanced-custom-fields/acf.php";
    i:3;s:45:"limit-login-attempts/limit-login-attempts.php";
    i:4;s:27:"redirection/redirection.php";
    i:6;s:33:"w3-total-cache/w3-total-cache.php";
    i:7;s:41:"wordpress-importer/wordpress-importer.php";
    i:8;s:24:"wordpress-seo/wp-seo.php";
    i:9;s:34:"wpml-string-translation/plugin.php";
    i:10;s:38:"wpml-translation-management/plugin.php";
}

I added a new line (for the missing plugin) and incremented the a:10 to a:11 to indicate that there are now 11 items in the array:

a:11:{
    i:0;s:49:"1and1-wordpress-wizard/1and1-wordpress-wizard.php";
    i:1;s:29:"acf-repeater/acf-repeater.php";
    i:2;s:30:"advanced-custom-fields/acf.php";
    i:3;s:45:"limit-login-attempts/limit-login-attempts.php";
    i:4;s:27:"redirection/redirection.php";
    i:5;s:40:"sitepress-multilingual-cms/sitepress.php";
    i:6;s:33:"w3-total-cache/w3-total-cache.php";
    i:7;s:41:"wordpress-importer/wordpress-importer.php";
    i:8;s:24:"wordpress-seo/wp-seo.php";
    i:9;s:34:"wpml-string-translation/plugin.php";
    i:10;s:38:"wpml-translation-management/plugin.php";
}

i: appears to be item number, and thanks to JHoffmann's comment, it appears s: is the length of the string that follows.

The site now works as before!

//Using this code you can activate your plugin from the functions.php
    function activate_plugin_via_php() {
        $active_plugins = get_option( 'active_plugins' );
        array_push($active_plugins, 'unyson/unyson.php'); /* Here just replace unyson plugin directory and plugin file*/
        update_option( 'active_plugins', $active_plugins );    
    }
    add_action( 'init', 'activate_plugin_via_php' );

Just another answer for a different approach that could benefit someone else in the future. You could also move the plugin folder to the Must Use folder (which you will probably need to create if not used before. This path is usually:

wp-content/mu-plugins

Plugins in this folder will always run. Refer to the following for more info:

https://codex.wordpress.org/Must_Use_Plugins

Note: The only thing to consider is that these plugins are loaded before others in the plugins folder. Also refer to the caveats in the above link as there may be other issues that could prevent your plugin working correctly.