Try restarting the phone. If that doesn't work. Try removing the SD card, restart the phone and reinsert the SD card. If the phone still doesn't read the data, try connecting the phone (while the SD card is still in it) to a computer via USB cord and see if the computer can access the data on the card that way. Should it not be able to, you then may want to try the card in another device and see if it is readable.
If none of this works, then your SD card may have been wiped. 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, Android-Recovery and Tenorshare. They are all data recovery apps/programs. There are many others out there, so check around and see which you would rather try. There is also one called RECUVA, that I have heard is free, which you could try.
One important note though, most of these programs are not free. They may say they are "free" or have "Free Trials" and "Free Downloads". Yet to actually recover your data, and get your files back, you may be have to purchase the program/app. Just so you know.
NOTE: For future reference, I really would suggest backing up your data to a computer or cloud. Often. Better to spend the time doing that, rather than to be out all that stuff if the phone dies, things get deleted/erased or it gets stolen.