Here is another reminder that no computer is truly "user friendly", not even the Mac.
My daughter couldn't login to the iMac anymore. The darn thing just hung up and didn't move. I'm lucky that I know how to
ps -auxfrom another computer so that I could see that it was hung up on this process:
/System/Library/LoginPlugins/MCX.loginPlugin/Contents/MacOS/MCXAppItems -u My Applications.
Plugging "MCXAppItems hang" into google found this hint, which worked for me.