After doing enough searching online, I was finally able to find a new host for the website's guestbook. You should be able to send in messages like before.
Apparently, 123Guestbook wanted to get an early start on their shutdown. Until next month, the Guestbook has been automatically set to read-only and will not accept any more submissions. Which means I'm going to have to look for a new Guestbook host.
Many people are moving to Atabook, if you wanna give that a try: https://atabook.org/