same SD card has worked for years
SD Cards will not last for ever, the more they are written to, the faster they wear out.
I fsck'ed the file systems on the card to ensure they were OK
An fsck will check the integrity of the file system, but not the integrity of the SD Card, did you try writing to it ?
some sort failure contact with the card
Certainly a possibility, it has happened to both me and others (search the forum).
Try cleaning the SD Card contacts with a pencil (not ink) eraser, you may have to shape it first with a scalpel.
I also use an old SD Card onto which I liberally spray contact cleaner and then repeatedly rapidly insert & eject from the plug in an attempt to clean the internal contacts.
Finally, you could clone the SD Card to a new one and then try that in the plug.