Everyone runs into trouble sooner or later. Here is some advice on how to handle the most common problems:
If an application is stalled or frozen — that is, if it does not respond to your mouse clicks and keyboard commands, you can either wait and hope that it wakes up, or kill it. If you decide to kill it (NOTE: You will lose all unsaved data), start the GNOME System monitor (from GNOME Menu ▸ Applications ▸ Sytem Tools). Select the application you want to kill and click on End process button at the bottom. You can also right-click on the application name and choose End process; if it doesn't help, right-click and choose choose Kill process. Using GNOME System monitor also allows you to find and kill all helper processes started by this application.
If a GNOME application freezes or crashes (unexpectedly dies) repeatedly, you should file a bug report as described in GNOME feedabck page.
If your whole system is frozen and is not responding, do not hurry to push the power button on the computer — this is usually the worst solution. Most probably, it is not the operating system itself that is frozen (UNIX systems are known for stability), but just the graphical part, X Window System. In this case, you can try to restart X Window System by simultaneously pressing Ctrl+Alt+Backspace. This should work for the implementation of X Window system used on Linux and *BSD, XFree86 (unless it was disabled by your system administrator). Of course, in this way you also lose all unsaved data, but at least you do not risk to mess up the whole file system.
If you have a serious problem with your GNOME settings — for example, if your panel is missing — the radical solution is to remove all your GNOME configuration files and start from scratch. This is an emergency solution, as you lose all configuration settings and will need to configure your menus, panels, etc. again from scratch, that is, from the default GNOME configuration. However, this only affects GNOME configuration, so your data files and settings for non-GNOME applications remain intact.
To remove all your GNOME configuration settings and return to the original GNOME configuration, logout then log in again holding down keys Ctrl and Shift (immediately after entering your password in the login dialog). You will be presented a dialog, offering you the choice to reset the saved session (that is, which applications were open when you last logged out); reset your GNOME configuration settings; or both.
Finally, if you have really severe problems and your system freezes or becomes otherwise unusable as soon as you login, you have one last option. You can select Failsafe session type instead of the default GNOME in the login screen. Both GNOME and KDE Login Managers support this. In this session type, GNOME is not started; instead, you are presented with a single terminal window. This is almost guaranteed to start OK, and if you know how to use command-line tools to find and fix your problem, you have a chance. Otherwise, ask an expert.