You can search for this bug and it seems like it may be related to ecryptfs and is many years old.
The symptoms are that you return to the computer and the screensaver was active or the screen was asleep/black and it doesn't seem to come back. But you check by SSH the computer is running fine and are frustrated you'll lose your running programs and have to reboot.
There is a simple solution:
Ctrl + Alt + F1
Ctrl + Alt + F8
Basically you are switching to another virtual console/screen and then back to screen 8 which is your Desktop. This removes the black screen and presents the login prompt and doesn't cause any loss of data or interruption to your session.
This also works if you are using your computer normally and the screen becomes partially dark or very dark. It seems related to the above bug.
It may also be related to a bug in the Intel i915 driver but this issue seems to plague Nvidia and AMD users too.
Some related syslog or dmesg errors you may see:
mate-screensaver-dialog: pam_ecryptfs: seteuid error
[ 4.825400] [drm] RC6 on
[15732.058803] [drm:intel_pipe_update_end [i915_bpo]] *ERROR* Atomic update failure on pipe A (start=172171 end=172172) time 100 us, min 894, max 899, scanline start 893, end 900
[24966.142220] [drm:intel_pipe_update_end [i915_bpo]] *ERROR* Atomic update failure on pipe A (start=54325 end=54326) time 101 us, min 894, max 899, scanline start 893, end 900
[252173.205297] [drm:intel_pipe_update_end [i915_bpo]] *ERROR* Atomic update failure on pipe A (start=33067 end=33068) time 102 us, min 894, max 899, scanline start 893, end 900
[266867.040745] [drm:intel_pipe_update_end [i915_bpo]] *ERROR* Atomic update failure on pipe A (start=19806 end=19807) time 102 us, min 894, max 899, scanline start 893, end 900
[266872.190787] [drm:intel_pipe_update_end [i915_bpo]] *ERROR* Atomic update failure on pipe A (start=20115 end=20116) time 102 us, min 894, max 899, scanline start 893, end 899
[266873.174149] [drm:intel_pipe_update_end [i915_bpo]] *ERROR* Atomic update failure on pipe A (start=20174 end=20175) time 100 us, min 894, max 899, scanline start 893, end 900
[370925.989870] [drm] stuck on render ring
[370925.995049] [drm] GPU HANG: ecode 9:0:0x85dffffd, in Xorg [1562], reason: Engine(s) hung, action: reset
[370925.995054] [drm] GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.
[370925.995056] [drm] Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel
[370925.995057] [drm] drm/i915 developers can then reassign to the right component if it's not a kernel issue.
[370925.995059] [drm] The gpu crash dump is required to analyze gpu hangs, so please always attach it.
[370925.995061] [drm] GPU crash dump saved to /sys/class/drm/card0/error
[370925.998026] drm/i915: Resetting chip after gpu hang
[370928.001884] [drm] RC6 on
Other errors when the screen goes dark (sometimes Alt F1 + Alt F8 will not fix it)
This behavior was seen on kernel 4.4.0-116-generic
Sep 24 07:09:36 queenlazina-desktop console-kit-daemon[1768]: (process:6292): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 24 07:09:38 queenlazina-desktop console-kit-daemon[1768]: (process:6294): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 24 07:09:43 queenlazina-desktop console-kit-daemon[1768]: (process:6297): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 24 07:09:49 queenlazina-desktop console-kit-daemon[1768]: (process:6299): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 24 07:10:00 queenlazina-desktop console-kit-daemon[1768]: (process:6301): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 24 07:10:01 queenlazina-desktop console-kit-daemon[1768]: (process:6303): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Sep 24 07:10:08 queenlazina-desktop console-kit-daemon[1768]: (process:6305): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
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