Yikes lawaynescott! This is a nightmare - I can't imagine suddenly losing my photos, I would be heartbroken. To be honest, without any action taken on your part or any other symptoms it's really tough to say what could have caused this. The problem could be caused by faulty software or third party applications, or memory management errors. I've never had this happen personally, but have heard that sometimes when older devices run out of storage, or aren't frequently restarted, things may be deleted - do you remember getting any kind of error messages about the memory being full?
I know it's been a few days since your initial post - have you had some time to comb all the folders in the phone to see if perhaps they were moved, rather than deleted? I definitely hope you've been able to recover them!
Hey there! I just wanted to check in and see how everything's going for you... please let us know if you have any questions!