Hello. Usually when this occurs, it is due to someone who has mistakenly deleted the history while in the Defects module. There are a few ways that the defect history can be deleted. It can be done through the GUI in ALM via Customization, the project database in Site Administration, or the project database on the database server. The defect history can be deleted in bulk within the application using the 'Clear History' method located in the 'Tools' drop down. When the 'Clear History' method is selected, all of the history for the defects within the current project are deleted. The defect history could also be deleted by running a query against the project database within Site Administration, but deleting data directly from the database is not suggested, as this could corrupt the project. I have also seen records deleted in ALM due to network related issues, security breaches or server crashes. Since you're not aware of any changes made in ALM, and there hasen't been any server, environmental or network issues prior to the date the defect history went missing, then that leads me to believe that one of the project users mistakenly deleted the history. You may want to check with all the users in the affected project to see if anyone may have done this by accident. I have tested this in my environment, and unfortunately when the history is cleared, it isn't inidicated in the server logs, and there aren't any entries in the audit data tables. When the history is deleted, the entries are removed from the tables with no trace of them ever being in there. So unfortunaltely, there really isn't a way to troubleshoot this issue.
As a workaround, you could try the following, if you're okay with using new defect ID's for the existing defects, and you can re-link the defects to the other entities. The best route to take would be to restore your project backup into ALM using a different name other than the affected project. Next, you will open two instances of IE, and access the affected project in one instance, and the project backup in the other instance. After you have accessed the Defects module in both projects, you will copy the defects from the project backup, and paste them into the affected project. This will assign new defect ID's with all of the previous history, but the links and any update history added since the previous backup was taken will be lost. Hopefully the project backup isn't too old. We recommend taking daily backups of your projects to reduce the amount of data loss when something like this occurs.
To prevent this from occurring in the future, you can restrict users from deleteing the defect history in the 'Groups and Permissions' within the Customization menu for each project. To do so, access Customization, via 'Tools'(Gear cog) drop down -> Select 'Customize...' -> Groups and Permissions (left menu) -> Select the user group -> Select 'Permissions' -> Make sure the 'Clear History' checkbox is not selected. I hope this helps!