How to unlock locked session?

I do not have the answer to your bug, actually I experience the same thing, but I found here a way to recover the situation without reboot the machine/lightdm.

In your tty1 (Ctrl+Alt+F1), as root, type loginctl unlock-session [id], where [id] is the session id you get by typing loginctl list-sessions.

If it doesn't work with the first ID, try with the other session IDs of your user account.

I am not able to add comment. here are some words on Ubuntu. I boot Ubuntu 16.04.1 without login, then I close the lid for going out, and when I am back, reopen the notebook, not able to unlock (light display manager is shown on up-right corner, only password to enter, no user name)

Ctrl+Alt+F1 brings tty1, login my account, then

sudo -i loginctl list-sessions
sudo -i loginctl unlock-session id

As in https://askubuntu.com/a/611611/485005, "If it doesn't work with the first ID, try with the other session IDs"

I think you can simply disable and change the default screensaver locker anyway. Firstly disable light-locker at [LightDM/Xfce] Power Management Preferences followed by:

$ sudo apt-get update && sudo apt-get install xscreensaver && sudo apt-get remove light-locker

then after reboot (restart X) I got the xscreensaver as default screen locker manager.

This worked for me:

sudo service lightdm restart 

I had similar issues getting stuck on "You'll be redirected to the unlock dialog in a few seconds" screen in xubuntu 16.04.4 after booting up from a suspended session.

The solve for me is to press Ctrl+Alt+F7

I sometimes have to repeat the above command, but eventually it will allow me to login normally.