Pls help me

I use OPPO A15, yesterday suddenly all my photos and videos from gallery were got corrupt, I mean when I clicked on any photo it was just a black screen and it says can't load image same with videos.

When I tried to download images or video on WhatsApp, it was saying Unmouted internal storage as disc drive or something.

When I switched it off and open it my gallery got fixed but the problem with WhatsApp still stands. Although it not saying to Unmouted anything now but I still think that the problem is still same .

I tried to look on YouTube and Google for solution and it was saying to go in storage and SD card and change the setting but I can't find SD card setting in my phone. Pls help