Ah yes, maybe the OS decided to hide it, I've come across this long ago and used a utility in 'Advanced System Care' to fix it, though you can do it through windows system too
Are you still using XP?
go to registry editor> Run
type and enter> regedit
Navigate to:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
single click on the last path in left pane (Explorer)
In the left pane, see if 'NoViewOnDrive' is there, if so delete it
You may need to reboot, not sure, may jsut work off the bat
If squeemish with the registry editor, try this>
go to> Run
type gpedit.msc
click ok
in the tree in the right pane, go to>
User Configuration -> Administrative Templates -> Windows Components -> Windows Explorer
In that...