How to recover lost pictures (not deleted) on samsung galaxy s4?

I have a Samsung Galaxy S4. Today, I went to a concert and took some pictures and videos, but not too many (50 max). The night before, I moved most of my pictures onto an external hard drive to make room, so the "camera" folder was gone because I deleted the pics once I moved them, which is normal.

However, when I got home and plugged my phone in to charge, it randomly restarted even though my battery wasn't dead (it was at least 80%). Later when I checked my phone, I saw that some of my apps were "empty," meaning that the icons were gray and I couldn't open them, and all the pictures I took earlier were gone (but the pictures I left on my phone the night before were fine). I realized that all of these apps were on my sd card, so I turned off my phone, took out my sd card & reinserted a few times. This fixed the app issue, but my photos are still gone, which is kind of annoying. Does anyone know why this happened and/or how to fix this so I can recover my photos? If it was unclear, my photos are stored on my sd card, I didn't delete any photos, and my phone recognizes my sd card.

Thanks in advance! Sorry for such a long post.

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