I was having the same problem (trackpad not working after upgrading the OS to Monterey). But I managed to make the trackpad work again by connecting a mouse and going into System Preferences > Accessibility > Pointer Control > Mouse and Trackpad and unchecking the option "Ignore trackpad when mouse or wireless trackpad is present". Unfortunately, it seems I'm unable to disable the trackpad again until I restart the system, and after a restart, the trackpad is disabled again and I have to go through the process above once more. Edit: ok, apparently, if I leave that option unchecked, the trackpad continues working after a restart, so I'll leave it unchecked for now.