Many people find session management very useful. However, sometimes session management can go astray, and do undesired things like remember your settings after your kid brother attacked your computer. If GNOME is properly installed, returning your settings to the default is easy.
First, exit completely out of X. If you are using session management, you should be able to use the Log out item of the Foot menu to exit. If that doesn't work, hold down the control and alt keys, and hit backspace. If that still doesn't work, and you are now looking at an XDM or GDM login prompt, switch to a text login (control-alt-F1 on many systems, consult your system documentation if you aren't sure, log in as a different user and use su in an xterm if you are really in a bind).
Then, to remove your settings, while you are out of X, remove the ~/.gnome/session file. The next time you start X, or the next time you log in from GDM, it should start you with the default window manager, panel, gmc and the help browser.
If you use session management, and it can't find a session, this is what it gets. First, have you tried resetting your session management settings (See above)? If this doesn't fix it, than gnome-session is having trouble finding the default.session file. In gnome-core versions 1.03 and later the file should be in $prefix/share/gnome. Prior to that it should be in $prefix/share. If the file is missing, you should probably reinstall gnome-core.
This is a tougher question than it sounds, since the answer depends on which version of GNOME you are running. The easy answer is to use the Window Manager settings in control-center. If that doesn't work for you for some reason, here's how to do it.
In gnome-core version 1.03 and earlier, the window manager is selected by placing it in a file called default.wm. The system default for this file should be in either the $prefix/share/gnome or $prefix/share directories, whichever one has the file default.session.
A user-specific version of default.wm can be put in the ~/.gnome directory. The default.wm file should look like: