What to do when system stops interacting with I/O devices?

Due to some applications misbehaving (which I have to force kill), there seem to be some secondary effects on my system. The screen output freezes, but the system stays responsive (I can ssh into the machine, and verify that no program is hogging CPU / memory). It gives the appearance of being "frozen" though, because moving the mouse or keyboard doesn't do *anything*. The keyboard's caps lock key doesn't lead to caps light toggling. Plugging the devices in / out doesn't help. However, the reisub sequence works to reboot the system, so perhaps the system doesn't stop receiving input, only the output. I'm mostly having trouble understanding where the issue could be. But I'm in a fortunate situation of being able to ssh into the machine and have full control over it, so I'd like to know what I should try next time this happens. System-specific details: Ubuntu 20.04 with default GNOME and X11.