Did you install the card as adoptable (internal) storage or portable (external) storage? If it's adoptable storage, it's seen as just larger internal storage, not as external storage.
If it's portable storage, put it into a USB adapter, plug that into a PC and see if the card is readable. If it is, try putting it back in the phone, but the SD port in the phone could be bad. If it's not, and it's a SanDisk card, go to
SanDisk microSD Tech Support, upper right on the page, and use Live Chat. It's probably still under warranty, so they'll replace it. If it's someone else's card, contact them and see if they'll replace it. (SanDisk's minimum warranty - for cheaper cards - is 5 years. Some of them carry lifetime warranties. Other manufacturers? Who knows?)