You lost all the space, or all the files? If it is space, that means it is full and you just need to remove files. If it is files that are missing, then you need to know how it was removed. Did you format the card? That would make them gone for good. The same goes for if you turned a SD card that had files already on it into "Internal" storage. Same effect and they would be gone for good.
If, however, you just deleted the files, well you 'might' be able to recover some or all of them. While there is no guaranty that you can get them back. Deleted does mean deleted. There are a few apps/programs out there that 'may' be able to help you recover the data.
A few of the programs you could try would be... DiskDigger, MobiSaver...