1. The card has malfunctioned.
2. Probably the memory card is out of cellphone supporting specification, such as MicroSD 64GB.
You may check your cellphone specification at first. This is because most cellphones support up to 32GB,
and 64GB are supported by high level cellphones, ex. smartphone. With 64GB memory card supported,
the default format is exFAT and may not match with some MicroSD cards formatted by FAT32.
And you need to re-format the card on your cellphone again