Thank you. Crash is perhaps the wrong word. The computer was working; the hard drive is OK. After rebooting (to deal with the computer going sluggish), I found that all the data entered after 2011 (July or so) were lost; that was the time when I updated the OS X to 10.6.3, and subsequently to 10.6.6. Rebooting always worked fine (shutting the power down, and then on). This time, after rebooting, I got a message from Google that "some plug-ins are out of date." I was alerted that the update requires OS X 10.6; but I didn't have it any more. By the way my browser is FireFox. Google advised me to update it and use Chrome; I kept FireFox; I just updated it to 16.0.2, following the instruction that this is how to continue with X 10.5, which I also learned that Apple is stopping to support. [I wish I had known this sooner}. It is not a crash, indeed, the loss of a massive amount of data (hidden somewhere, no doubt).
I think it was shortly after summer 2011 that I found the Time Machine was full, and I was advised to get an external drive by the Apple Store and got the G-Technology's G-drive for further backup, 2 Terabytes. Now, trying to open it, I got the message "The Alias G-drive could ot be opened because the original item cannot be found." This was followed by another "Insert the disk or connec to the server volume and wait for it to appear on the desktop then try again." Nothing happened when the disk appeared on the desktop. I'm afraid re-installing this external drive may erase what is supposed to have been backed up. What is saved on the Time Machine is only the very old stuff.
Since the computer is now 5 years old, I am considering getting a new iMac; but I have to find a way to save the files and documents intact so that they can be transferred on to the new computer.