RecentX stores data under the folder %appdata%\Conceptworld\RecentX on Windows. Open this folder in Windows Explorer to know the exact location.
You can backup files in the above folder.
However, RecentX shouldn't be running when backing up this folder.
Actually, when every thing crashes and you reinstall RecentX , it will re-index all your files & folders again automatically. In most cases you don't need to take back up of RecentX files. When everything is gone, the location of new files & folders would mostly be different. So it is better to let RecentX re-index everything and show you new files list.