My SD card files disappeared after mounting to a different phone

I recently acquired a replacement phone and as soon as I mounted my existing SD card, the files disappeared while the SD card is showing the storage is in fact used. When I looked at it further, it seemed that Android recognized the files as 'system'. Now, even mounting it on Windows through an SD card reader doesn't work. However, if I use the MTP USB storage mode, it does show but without the files (just shows an empty SD card). Is there a way to undo or recover these files?