I just updated my Xubuntu 14.10 desktop to 15.04 and I'm having the following issue:
When I lock my computer (or when the inactivity timer locks it for me), the screen goes blank and I cannot get into my session. The power light on the monitor is still green which means that the monitor hasn't gone to sleep - i.e. that there is still a video signal but no picture is being sent. The only way to get back into the desktop environment is to switch to a text terminal (Ctrl+Alt+F1) and reboot.
I found a known issue about black screens after boot, but that's not my problem.
I searched through the syslog, but could not find any messages that seem to be related.
I have all the latest packages available via apt-get upgrade.
How can I troubleshoot and fix this issue?
I had the same problem. I fixed it by going into Light Locker Settings (not sure what it's called: I'm in Windows) and resetting the lockscreen option (turning it off then on again).