Sleep mode doesn't work

Hello,

I've bough a Dell Inspiron 1564 with win7 home premium x64 last month and everything was fine until a few days ago, when sleep mode stopped working. When I close my lid and open it up after some time there are 2 things that might happen:
1. The screen is just black, all the buttons are lit, I hear the fan, but no reaction at all. So I just forcibly turn off and turn it on again.
2. After opening the lid, nothing happens, when I try to turn it on, it loads up, but informs me that windows has not shut down properly last time (or something like that).

Thanks