Bob,
While I don't know what caused the problem, I have heard that a "Safe Boot" is often the cure:
1. Restart the computer (using the Power button or Reset/Interrupt button if necessary).
2. Immediately after the system startup sound, press and hold the Shift key until "Safe Boot" appears. Enter your password when asked.
Note: There may be a considerable delay, particularly on larger startup volumes, since the disk check and repair take place before "Safe Boot" appears.
3. After the system is fully started up, restart again normally.
HTH