1. If you inserted a card that is to large, more than 64GB it may not register it. All devices have a size limitation.
2. If the card is bad/damaged/used, it may not be read by the device.
3. The card slot in the phone could be damaged or non-functioning.
I would start by confirming the card storage size and also make sure it is the right type of card for your phone. Not all cards work in all phones. Then, should that no resolve it, try using the card in a different device and see if it works. If it does, then it is likely the phone. However, if the card is not readable by other devices either, then the card itself may be bad/damaged. I would then try a new card. And if this is a used card, I would suggest starting with a new one. Cards are easily damaged.
Should nothing above resolve it, then you may need to have the phone looked at by either the manufacturer, if it is under warranty, or by a local tech if it isn't.