There is no internal memory card. Internal memory comes built in to the device. Not a card. External memory, which on some devices can be changed to become internal memory, is through a SD Card you purchase and install in the device.
If you are referring to the SD Card, then it could be complicated. If you just unmounted the card, all you have to do is remove it from the device, restart the device (while the SD Card is out) and then once it has restarted completely, reinsert the card and it should allow you to remount it.
However, if you had the card changed to be "Internal" rather than "External" storage and then unmounted the card, that is an issue. When it is made internal it is encoded so that it only works with one device and it...