This suggests your storage card hasn't initialised properly, and the OS has assigned a 'ghost' name to it. This can be dangerous to your data
First thing to do is take the card out, and see if you can read it in a card reader on the PC- if so, make a full backup by copying the whole card to the PC. Now reinsert teh card into the WM device, and see if it sees our normal contents again. If not, take the card out again, and do a soft rest; if that doesn't work, leave the card in place, and do a soft reset. ( If the card wouldn't read in the PC, I'm afraid it's contents are probably corrupt- you could try a data recovery program.)
If it still isn't back, the the options become more complex! You could try a program called Pocket Mechanic to repair the card, otherwise it may be time to reformat the card, and copy back the backup files to it.