No idea what's wrong, possibly only a software issue but trying disabling hibernation. Quickest way to do this is to open an elevated command prompt and type "powercfg -h OFF" (minus the quotes). Have a browse of the Dell webshite to see if there any patches for this issue on this model. Power/sleep/suspend has been a pain in the arrse on more than one brand for a decade or more.
EDIT: Just checked, there's a BIOS update dated 29/9/2011 for that model to adress AMD PowerExpress issues. You don't say what your submodel of the machine is, but go to the Dell site, select support and enter the service tag printed on the bottom of your laptop to make sure you get the right updates.