I'm having the very same problem, in fact, it's the very reason why I joined the forums.
When I close my laptop,
it goes into hibernation and turns back on when I press the power button without a problem.
If I leave my laptop open, it will eventually enter sleep mode.
My old computer, which ran on Windows XP would do this too, but it would come out of sleep mode after hitting a key.
But with my laptop, which is running on Windows 7,
it does not come out of sleep mode and I have to do a hard reset to get it working again.
I think I read somewhere that this is a W7 bug, and there may be a patch for it,
but I'm not sure where I can find this patch.
For now, I am disabling sleep mode and automatic sleep mode by going to:
Control Panel > Hardware and Sound > Power Options > Edit Plan Settings
Then, I set the two drop boxes next to 'Put the computer to sleep:' to 'Never'.