Try removing ~/.kde4 (The .kde folder in your home directory.)
You can't normally see it unless you turn on "Show hidden files and folders" In the dolphin file manager.
After you have deleted .kde4 you will need to logout and and then log back into kde4.
There isn't a way around it but all your kde 4 settings will be reset after doing so.
If your plasma workspace is so badly corrupted that you can't do anything in kde 4 :
Use Ctrl + Alt + Backspace to reset the x server until the x server is no longer up. (You may instead be able to login using the kdm login screen into an xterm session and use as root the code : init 3)
It depends greatly on what distribution you use.
Next you are at a login prompt, so login as your normal user account and use the code :
rm -rf $HOME/.kde4
Next you can reboot the machine using :
Now the x server should have started as normal, if you then login to a kde 4 session, you should have reset your configuration for kde 4 and have a freshly created working environment.
Problems like this that you mention occur most frequently :
After you upgrade a quite old version of kde 4
You have found a bug by creating an unsupported configuration and could possibly be attributed to old plasmoids.