How can I send email if the security string won't let me fill in the letters?

Recently yahoo has chosen to put up a spam block when I try to send an email. Type the letters in the string. Fine, but it doesn't allow me to fill in the letters. I click the box, but nothing. I type the letters, nothing appears. I have changed settings. I have never encountered anything like this before. Is there someone who can help me out there?|||You can try calling Yahoo 866-562-7219 M-F 6-6