How to restore deleted CSV files?

Not really an Excel related question.

Create backups before you change anything.

There is software out there that will restore any deleted files, e.g. "Recover My Files" though this will depend on what actions you've performed since the deletion.
Once a file is deleted from a drive the space it took up becomes available for the system to write to. If you keep the drive the same and dont copy over any further files the deleted file should be available for restoring. But if you copy files to that drive it will use any available space and therefore could overwrite files that have been deleted. Then you'll never get them back.

Again backup files up occasionally is the best route to take.