This seems a bit odd for sure. Normally if you have comments enabled, you will get a new comment form on any post with comments.
Enabling comments is usually controlled by Dashboard -> Settings -> Discussion. Did you accidentally check the box to “Automatically close comments on posts older than xx days”? Or perhaps the “Users must be registered … “?
Do you have comments enabled in the other settings? Have you recently installed any comment handling plugin?
Comment: Been a while since I’ve seen that old Wheat header image. I’d guess you’ve been using Weaver and updating for some time.
That was it! “Automatically close comments on posts older than xx days” was checked. Seems very strange I would of accidently checked that. It instead might of happened when I switched my theme to Weaver Xtreme a few months ago (although that seems strange too). I started with that wheat header image over 10 years ago with the first version of Weaver 🙂