If you have to reboot your entire system, use the reset button on your computer. You will probably lose some of the file you were working on. The Macintosh system software does not currently multi-task. In other words, when one application crashes, the whole system goes with it. After any system freeze or hangup, it's a good idea to run Disk First Aid or Norton Disk Doctor.
Files often become corrupted in a crash.
Your Mac will run better if you repair damage right away. If you can, try clicking on the desktop to see if you can get out of the hung application to the Finder and shut down properly. Click OK or press return. If your Mac gives you an error message while you are in an application, there should be a restart box to click on.