If you still have filesystem access through your host, possibly through FTP, you can rename “piklist” to something like “piklist-old” in wp-content/plugins/ to deactivate it.

As for why you are experiencing this issue, it looks like your hosting environment is missing the PHP “mbstring” Multibyte extension. Here is some more information about installing it: http://www.php.net/manual/en/mbstring.installation.php

If you do not control your hosting platform, I suggest opening a ticket with your host and requesting they install the mbstring extension.