File permission error.

Hello,
Recently I have lost two very important files when I tried to save and exit my work. The message I got was "Word cannot complete the save due to a file permission error." I was asked if I wanted to try again, "yes", and a file was save but it was unreadable. Instead of letters I got a series of little squares and the file would freeze, meaning I couldn't scroll down or exit immediately. A real word would appear now and then but essentially the file was unusable and lost.
I am using a Microsoft Office 2000 professional program.
What is happening?