I just had this happen to me today. I spent a lot of years in computers and I found (especially living in a damp climate) that connection problems were usually the cause. I powered off my phone, took the SD card out wiped it clean and reinserted. Works fine now. Of course the newer photos which went wonky were no good but newly shot photos now work and all the old photos are still fine. Backed everything up.. Also all of my 8 year old memory cards for my professional video camera failed and were unusable when I took it out to record a Remembrance Day service for the local legion so beware that memory cards have life spans. Be sure to back up your photos.
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