Stopped reading my SD card? Why?

If a hard reset does not work, and your SD card still can not be read, I would try a SD card reader for your computer. Just to make sure that your card is okay. That way you can also save all your files from the SD card to your computer. Try a different card in your reader and see what happens. Worst case scenario. you will have to disassemble your phone and take a look at the SD card reader. Make sure the flex cable is still connected. The SD card reader is replaceable by itself, no reason to start looking for a new logic board. It is available at places like this and is not very expensive. The teardown is right here, and even so it is not a guide, it is a pretty good indication of what you are up against. hope this helps, good luck.