If you've recently changed computers, make sure that Steam Cloud is enabled on both the old and new computer. Steam Cloud cannot pull your save data unless both are enabled. You can also transfer your local save data files to your new computer. It is found in the path %APPDATA%\Arrowhead\Helldivers\saves\
If you've reformatted your computer, you should have had Steam Cloud enabled and your local data backed up. Make sure Steam Cloud is enabled in the settings after reformatting. Arrowhead unfortunately cannot restore players lost progress.
Your data is stored in the PSN Cloud and syncs automatically whenever you go online.