My HP Zbook15 laptop can go to Sleep without a problem right after powering up and login to Windows 7. However, if after a normal working day, when I put it to Sleep then the screen will go blank but the Power LED will stay on, the disk access LED faintly pulses, and the machine will not go to Sleep completely. I have to force a shutdown by holding down the Power button for a while. After powering up again, the DOS screen prompts me to boot Windows in Safe Mode or Normal Mode. Normal Mode works OK. There will be a small window showing some error message about the last shutdown and invites me to "find more information", but when clicking that button, nothing happens.
So, in case a Zbook laptop fails to go to Sleep, what are the general debugging steps?