In Finder's Menu, select Go menu>Go to Folder, and go to "/volumes". (without the quotes)
Volumes is where an alias to your hard drive ("/" at boot) is placed at startup, and where all the "mount points" for auxiliary drives are created for you to access them. This folder is normally hidden from view.
Does it show there?
Safe Boot , (holding Shift key down at bootup), use Disk Utility from there to Repair Permissions, test if things work OK in Safe Mode.
Then move these files to the Desktop...
/Users/YourUserName/Library/Preferences/com.apple.finder.plist
/Users/YourUserName/Library/Preferences/com.apple.systempreferences.plist
/Users/YourUserName/Library/Preferences/com.apple.sidebarlists.plist
/Users/YourUserName/Library/Preferences/com.apple.desktop.plist
/Users/YourUserName/Library/Preferences/com.apple.recentitems.plist
Reboot & test.
PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive.