Why won't my laptop wake up when I close the lid and it goes to sleep after a few hours?

I recently bought a new Dell Inspiron 1521 laptop with Windows Vista. My settings are such that when I close the lid of the laptop (and its not shutdown), it goes to sleep. If, for example, half an hour later I open the lid, the computer wakes up and is back to normal no problem. But if I wait a few hours and then try to open the lid, the computer does not wake up. I push the power button to wake it up, but then a black screen with a blinking cursor comes up. At this point, I can't wake the computer back up, and I have to hold the power button and shut it down, and then start it over again. How can I wake it up in this situation?