Welcome to Android Central! In Settings>Storage, is there an actual button for Mount SD card? If so, tap it--does it show up now? Otherwise, remove the SD card, then insert it again and make sure it is well-seated. Does it register now?
If not, then remove the SD card and insert it into your computer (using a fullsize SD adapter). Does your computer read it? If not, it may have been corrupted by the phone. This is not an unusual complaint that we encounter here on AC.