when upgrading to Libra4.0, my entire book that i was writing is gone. there is no documents in the new version. Can I revert back, and will my files be there?
I feel that @mahfiaz answer is already a good help. If you are using windows you can use the search function. Right click on your drive where data are stored, select SEARCH, enter the file name or parts of it and let the search run. Other OS should have similar systems but I cannot explain them.
It looks like you depended entirely on “Recent documents” list. The easiest way to find your files is search for the name of your book file (the name it showed up in the list of recent documents). http://lmgtfy.com/?q=windows+find+files