Hopefully, there are backups. Had they disappeared when you started the browser again or did they vanish while the browser was running? In either case, do you see a list of backups in the bookmark manager? ctl+shft+o will get the manager/library. At the top you will see Import and Backup with a down arrow. Click on that arrow and you will see Restore with a right-pointing arrow. The backups are listed in descending order chronologically.