A few months ago the system I used to filter out spam, Mollom, was discontinued. I signed up with Mollom pretty early on, and it has been a great tool over the years.

I didn't realize that my website was unable to accept comments until Gerben pointed it out -- thank you! -- and the retirement of Mollom turned out to be the issue.

I just switched to another tool called Honeypot, let's see how well that does, and comments should be working again.