It sounds like you have removed the retention bracket that holds the heatsink in place.
The retention bracket screws into a back plate the other side of the mobo, see pic.
If this has slid away the only way to re align the screw holes is to remoce the mobo from your case and screw it back together.
When you need to remove your heatsink you don't need to unscrrew the retantion plate you just release the clips that hold the heatsink on it.
Lay the machine on its side, unplug and remove all components and then the mobo, you will then see the bacckplate and undersatnd.
And if you then need to re-apply thermal paste to your heatsink this will help.http://www.arcticsilver.com/amd_application_method.html#