Absolutely right here. Personal experience here on multiple occasions. I usually don't quote someone just to +1 it, but I feel it needs stressing. Xandros9 is 100% correct here. Take it out, get a new card for the phone. Use a PC to attempt recovery using a tool like Recuva. If Recuva can't get it, then you probably won't be able to recover it.
More than likely, from the sound of things, with the "deleted" files reappearing and "new files" disappearing, you may find that those files were only deleted and added in the phone's cache, and were never successfully written to the card. The card has most likely had the Input side of the Input/output controller borked. In all likelihood, this is a case of another SD card entering what is known as "permanent read-only mode" where you won't even be able to format it with a PC. It renders the card useless, if that is what has happened.
First thing, try the Recuva on PC. If that doesn't get it, test writing to the card from PC. If you can't write to it, test deleting a file. If the files won't permanently delete, try the Format option from the PC, and if the card fails to format, it's gone for good.
Good luck, and let us know how it all turns out.
Oh, don't buy the same brand and model SD card to replace. Switch brands, because certain brands have this issue quite often with Windows Phones. Team Group cards tend to have the issue on their less expensive cards, for one example. SanDisk is usually okay if you get the Ultra models. Not that others aren't good, but generally speaking I haven't yet heard of a problem with the SanDisk Ultra.
According to the tech support directors of 3 different companies I have researched this very issue with, there is actually a compatibility issue between some phone SOCs and the memory controller for the cards. The cards were designed with cameras in mind first, and apparently some of these cards have issues with phones using several of the Qualcomm SOCs.
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