Access to C: denied.

I dont know what really happened but I cannot access my C: It keeps on saying C:\ is not accessible. Acces is denied
I also cannot copy anything on my desktop or folders onto external drives and USB.

When I checked on the properties of C:. it is 0 bytes.

I am lost here. Some one please help me.

I have stuff on my computer that I dont wanna lose.:confused: