When a website asks you to copy words into a box to prove you're a human being, that's a CAPTCHA. The term stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart. Blogs and websites set up CAPTCHAs to screen out computer programs trying to spam them. Sometimes, however even human users have CAPTCHA issues.
The CAPTCHA Obstacle
When a website or a blog has a CAPTCHA box set up, you have to follow the instructions -- add two numbers together, or retype the text -- before you can interact with the site. Sometimes the box holding the text comes up blank, which makes it impossible to proceed. For instance, the LinkedIn professional networking site mentions that problems seeing its CAPTCHA box have prevented some LinkedIn users from accessing their account or setting one up.
Enabling Images
Sometimes the fault lies in what you authorize your browser to do. LinkedIn says that Internet Explorer security settings can screen out CAPTCHA text if the browser isn't enabled to access "mixed content." Similarly, Mozilla says if images aren't enabled on their Firefox browser, CAPTCHA boxes may not appear. Adjusting the settings on your browser may solve the problem. If not, switching from the browser you're using to a different one may make the CAPTCHA text visible.
Extension Issues
Sometimes the problem lies in your extensions, the add-ons you attach to Internet Explorer, Firefox, Chrome or other browsers to make them work better for you. Extensions are useful, but they can have unexpected side effects, such as preventing the browser from showing website images. Each browser has its own method for determining if there's an add-on issue. For example, you can set Firefox into "safe mode," which allows you to test how it works when stripped of add-ons.
Sightless Users
If you see a wheelchair icon next to the CAPTCHA box, you can click on the icon to get an audio version of the CAPTCHA test. The official CAPTCHA website recommends that all sites using CAPTCHA have an audio alternative so that visually impaired and blind computer users can get past the test. Fully sighted users can also use this option.