Have trouble with your Mac hanging up during boot, or have weird application problems you’re not quite sure how to fix (like jEdit all of a sudden stops working)? Try booting into single-user mode!

1. Reboot, and hold down the Apple key and S.
2. When you finally get a command prompt, type: fsck -y and hit Enter.
3. When it finishes, type: reboot and hit Enter

If that doesn’t fix, then you’re in real trouble and need to seek professional help.