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.
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.
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.