This is usually the case when no keyboard input has been made for a long time. Then it is usually sufficient if you press the up or down arrow key once so that an image is displayed again.
It is seldom necessary to restart the VNC service.
We have explained how you can do this here .
But it’s also possible that you’re still seeing the wrong console.
This is often the case with Ubuntu.
Then you have to change the console with the key combination ALT + F1 or ALT + F2 , and you have the usual display again.