Use a tool such as OmniDiskSweeper to explore your volume and find out what's taking up the space.
Proceed further only if the problem hasn't been solved.
ODS can't see the whole filesystem when you run it just by double-clicking; it only sees files that you have permission to read. To really see everything, you have to run it as root.
First, back up all data if you haven't already done so. No matter what happens, you should be able to restore your system to the state it was in at the time of that backup.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ If you’re running Mac OS X 10.7 or later, open LaunchPad. Click Utilities, then Terminal in the page that opens.
Drag or copy — do not type — the following line into the Terminal window, then press return:
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up.
I don't recommend that you make a habit of this. Don't delete anything while running ODS as root. When you're done with it, quit it and also quit Terminal.
