32GB Sandisk Micro SD suddenly stopped being recognized by Galaxy S3 phone

Until a few days ago my Sandisk card was working perfectly. Then I got a message on my Motorola Moto E 2nd edition phone indicating that the SD card was not inserted. I’ve been getting this indication intermittently since then. I can read the card on my laptop; it shows what looks like a standard set of files dated 11/30/16. I can think of three possible causes: a software defect, a defect in the card itself, or a bad contact between the card and the socket I’ve removed and reinserted the card several times, and also tried cleaning the card’s socket with an eraser. Does anyone have a suggestion as to how I might determine which of the three causes applies, and if so, what I can do about it?