DO NOT ATTEMPT TO WRITE ANY DATA TO THE CARD.
remove the sd card, and plug it into a pc. Check to see if the videos folder is showing any data volume (for example, there is 4Gb of unaccessible video data stored there). Do likewise for the photos folder. If there is data there, you should now try to recover those files with a recovery tool, such as Zero Assumption Recovery (ZAR). Do a DEEP SCAN of the sd card, moving any recovered files temporarily onto your pc. You can copy them back later. Once you are confident that you have recovered all of your "missing" files, place the sd card back into your phone, then format it. Once you have done all that, drag'n'drop the recovered files back onto the card. Be sure to backup the recovered files elsewhere, such as to OneDrive, a separate USB stick, etc.