I'm using an HP G62 laptop with Intel HD Graphics card and an Intel i3 Quad-core processor.
When I first boot my laptop, I can login just fine and work with the system.
The problem begins when I lock the PC, when I leave it for a few minutes and return to work. The login box appears, I type in my password and press enter, the screen goes blank and all I can see is the mouse curser.
It will hang there until I either do a hard restart (holding down the power button) or I log into tty1 and restart lightdm - then the log in screen comes back and I can log in again, but all my previously open programs are closed and all unsaved data is lost.