To check the capacity of a PlayStation Vita memory card, go to the PlayStation Vita system's LiveArea and choose Settings > [System] > [System Information] > [Capacity].
Make sure you have inserted an Official memory card (designed specifically for PlayStation Vita).PlayStation Vita can only use officially licensed memory cards.
If you've checked that you're using the correct memory card, turn off your PlayStation Vita by pressing and holding the power button for two seconds and then tapping "Power Off". Once your PlayStation Vita's turned off, try taking the memory card out of the console and putting it back in.
Note: Make sure that you don't take the memory card out while the PlayStation Vita is still turned on. If you remove or insert a memory card while the system is turned on you could lose some data.