I regret to announce that my web contact form was malfunctioning for an extended period of time. If you used the web form to try to contact me and didn't get a response, please try again. For the curious, some details follow.
background
The company that hosts my server made some changes to its data center, including switching to new DNS servers. Unfortunately the e-mails they sent didn't mention the new DNS servers, so while I had updated my server's other network settings to match the new data center configuration, the old DNS servers were still in use.
This resulted in a total DNS failure, which did not affect web serving, but did prevent the contact form from sending. The e-mail backend did not report any errors to the web server, so the "Success" page was still displayed (albeit after a lengthy delay), but the message was silently discarded.
resolution
The problem was fixed by updating the server's network settings with the new DNS servers, including a lot more backup servers in case multiple DNS servers go down. I will also be considering ways to simplify and strengthen the contact system.