Right or control click the MacintoshHD icon on your Desktop.
Click Get Info. In the Get Info window you will see Capacity and Available. Make sure there's a minimum of 15% free disk space.
If your Mac is running v10.7 Lion, click here > OSX Tips The Storage Display
If you aren't sure which Mac OS X is installed, click your Apple menu icon top left in your screen. From the dorp down menu click About This Mac.
You'll see Mac OS X version ............
As to your HD not showing on your desktop in the Finder:
Go to Finder>Finder Preferences and under General make sure that your prefs are set like this:
Then you'll be able to see everything on your desktop and you can either "Get Info" on your HD, or use the "About this Mac" from the Apple menu and go to "More Info" and look at the Storage tab on top.
I'll let everyone else hash the rest out...