Hi,
There are two possibilities:
1. The data is still there, but there is a query crashing which prevent the system to load the data. Turn on the debug mode of the joomla configuration and try again. If you get a MySQL error, then it means that you are in that case and knowing the MySQL error should allow us to tell you what to do to fix it.
2. One of the mass actions you run deleted the data in the database. Unfortunately, the only way to get it back will be to restore the data from a backup of your website. Mass actions are powerful but can be dangerous. It's best to first try them on a copy of your website before setting them up on your live website.