I had something similar.
I used to lock the session every night and let the computer torrenting, and unlock it the next morning. A few times, it seemed to not react to any input, and i could only hard reboot. Until the day i upgraded my hardware, and my monitor decided not to switch on, forcing me to buy a new one on the spot.
Also, i just spotted that Xfce has an issue that any monitor that disconnects is removed, and cannot display anything until “reconfigured” or rebooting.