The ssh connection is normal. After Ubuntu restarts, there is no signal when connecting to the display.

problem:

The Ubantu system cannot connect to the monitor after a power outage and restarts. Neither the dp socket nor the hdmi socket can connect to the monitor, and both show no signal. 

So after surfing on Baidu, I can’t restart anything, and I can’t unplug and plug the power cord. Finally, I searched for related information and found relevant solutions. After repeated trials, I don’t have to worry about a black screen anymore, hahahahaha

Be careful not to unplug or plug the monitor cable

The repair process is as follows:

sudo service lightdm stop

Stop first, after the execution is complete, then execute

sudo service lightdm restart

Be careful not to unplug the monitor cable

Then move the mouse or something to see if the monitor lights up?