MULTIPLE error messages

Hello all, I hope you’ll be able to help me.
I have been using a ww.wp.xz.cn blog since september 2013 and for the past few months it’s been showing error messages either when try to access to /wp-admin or when I want to update plugins.

I have manually upgraded my site through FTP following the instructions given by WordPress to make sure I have a “fresh” base, but I still get these messages every single day. Here are a couple of examples of the different messages it’s showing, does anyone know what could be the issue?

Warning: require(/flex/domain/MyWebsite.com/site/www/blog/wp-includes/query.php): failed to open stream: Input/output error in /flex/domain/MyWebsite.com/site/www/blog/wp-settings.php on line 116 Fatal error: require(): Failed opening required ‘/flex/domain/MyWebsite.com/site/www/blog/wp-includes/query.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /flex/domain/MyWebsite.com/site/www/blog/wp-settings.php on line 116

Warning: require_once(/flex/domain/MyWebsite.com/site/www/blog/wp-admin/includes/user.php): failed to open stream: Input/output error in /flex/domain/MyWebsite.com/site/www/blog/wp-admin/includes/admin.php on line 60 Fatal error: require_once(): Failed opening required ‘/flex/domain/MyWebsite.com/site/www/blog/wp-admin/includes/user.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /flex/domain/MyWebsite.com/site/www/blog/wp-admin/includes/admin.php on line 60

Warning: require(/flex/domain/MyWebsite.com/site/www/blog/wp-includes/comment.php): failed to open stream: Input/output error in /flex/domain/MyWebsite.com/site/www/blog/wp-settings.php on line 133 Fatal error: require(): Failed opening required ‘/flex/domain/MyWebsite.com/site/www/blog/wp-includes/comment.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /flex/domain/MyWebsite.com/site/www/blog/wp-settings.php on line 133

Warning: require_once(/flex/domain/MyWebsite.com/site/www/blog/wp-content/plugins/jetpack/modules/contact-form/admin.php): failed to open stream: Input/output error in /flex/domain/MyWebsite.com/site/www/blog/wp-content/plugins/jetpack/modules/contact-form/grunion-contact-form.php on line 17 Fatal error: require_once(): Failed opening required ‘/flex/domain/MyWebsite.com/site/www/blog/wp-content/plugins/jetpack/modules/contact-form//admin.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /flex/domain/MyWebsite.com/site/www/blog/wp-content/plugins/jetpack/modules/contact-form/grunion-contact-form.php on line 17

Warning: require(/flex/domain/MyWebsite.com/site/www/blog/wp-includes/author-template.php): failed to open stream: Input/output error in /flex/domain/MyWebsite.com/site/www/blog/wp-settings.php on line 125 Fatal error: require(): Failed opening required ‘/flex/domain/MyWebsite.com/site/www/blog/wp-includes/author-template.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /flex/domain/MyWebsite.com/site/www/blog/wp-settings.php on line 125