Something is probably running in the background which shouldnt be! It might be trying to install a Windows update, or run a virus scan. Or sometimes a wireless connection will cause problems. Try turning the wireless off (on the laptop) and remove any USB devices.
If this doesnt work you can force it to shut down by holding the power button in for a few seconds.
...and when you start it up again, run all the Windows updates/virus scans, just in case it is a virus!