The issue occurs not that often. You lock your screen, then get back in few minutes and see a login screen. The problem is that the login screen misses an important part: password input field.
I got this issue 2 or 3 times in last two month. There is a corresponding Launchpad entry. The bug has "Fix released" status for Ubuntu since mid September. Yet somehow it still happens. Last time I saw this bug I had Ubuntu 14.04 with unity 7.2.2. Apt-get-upgrading changed version of unity to 7.2.3. So, maybe it is indeed fixed now. However, if you encounter this bug try following steps:
- Switch to console (usually you can do this by pressing Ctrl+Alt+F1).
- Login if you haven't done this earlier.
- Kill all instances of unity-panel-service process (sudo killall unity-panel-service).
- Switch back to graphical interface (Ctrl+Alt+F7).
- If problem persists, repeat one more time.
A few notes. After switching to graphical interface it might be that you first see a black screen. In that case wait for some time to let the service to restart. In case of success you will be able to unlock the screen. Also you would find that applications are no longer on the virtual desktops that they were positioned initially. So, you will have to position them one more time manually.
In the comments to bug report there was one more solution. You could try running unity --replace. In my case that worked, but all windows lost their headers and borders. Probably, it could be sorted out. For example, by starting metacity, but it seems like a kinda longer workaround.
UPD. Had this issue again a few minutes ago. Seems like this workaround doesn't always help.