Unplug the power supply. Remove the battery. Press the power button and hold it for 10 seconds. Now, plug the power supply back in, and without installing the battery, press the power button and see if it boots.
if you are not going to use your computer for several hours, as in your case, it is far better to turn the computer off versus letting it sleep. Sleep is meant for short periods. If you do not wish to turn it off, consider enabling hibernation and hibernating the unit instead. Your best option, though, is to turn the computer off when you go to bed. Sleep mode does maintaing power to the system, and data corruption can occur (which is probably what happened in your case).http://h30434.www3.hp.com/t5/Notebook-Hardware/My-HP-laptop-is-all-of-a-sudden-not-booting-up/td-p/42539http://supportforhp.iyogi.com/laptop-not-booting.html
Hope this information helps you.