Since Windows 7 has a lot more folders (the Libraries), you may need to use the scrollbar on the left hand side and scroll down to locate the drive letters.
I have the same problem. Sometimes closing the explorer and reopening it corrects the problem. Sometimes I have to re boot to get the drives back i the explorer window or the computer window. The top line of the display moves from left to right as if the program was looking for drives and can not find any to display. The favorites and libraries sections are also empty.
Diskmgt.msc shows all of the drives as they should be.