I had last night, as Yoda might say.
I couldn't restart or shut down my MacBook Pro or log out. Using Disk Utility, I tried to repair the permissions and verify the hard disk. Upon trying to perform the verification, I got a message saying that I needed to repair the disk. I didn't have the installation disk for the laptop's OS, but I had one for a slightly older OS. After several attempts at rebooting, I was unable to continue. The Apple icon would come on the screen, but the laptop would beep and the log-in window would not appear.
Using my Nook, I made a reservation at the Genius Bar for this morning. After I convinced the technician there was a problem that required his attention, he took the computer in back and opened it up. When he brought it back to me, the log-in window was on the screen, and he had me sign in to make sure the Finder window would appear. It did, accompanied by a message saying that the clock needed to be reset. I asked him what had caused the problem, and he explained that the OS was having trouble finding the RAM, probably due to a particle of dust or debris somehow getting inside
I have just reset the clock and reconnected to our wireless network, and all is right in my tiny corner of the universe.
Rereading what I've written above, I realize I made a mistake in trying to reboot from the installation disk for another computer. I now remember being told by an IT person that installation disks for iMacs cannot be used for MacBook Pros.
Anyway, my thanks to Derek of the Genius Bar for putting a smile back on my Mac :-)