The iTunes Library file cannot be saved

Hi, I use ITunes for my iPod player. When I want to save some libraries, I meet an error. The error message box appeared on my screen, it said that it cannot save the library, herewith my post is the error message screen shot.
ITunes
The iTunes Library file cannot be saved. An unknown error occurred
When I look upon the error message, it is not clear to what process has caused the error, since it did not state the cause of the error, this being my first time to see such an error.
I restarted my computer to refresh the memory, but same thing happens, even when I tried saving the library.
Please share some ideas on this issue I posted, many thanks.
