Hi naoki and welcome to the Q&A Forum,
It's generally recommended that you delete images from a memory card and once and a while format it. Use the camera itself for erasing images (either all or individually) or formatting rather then delete them.
I don't know exactly what caused your problem, but I suggest you do an in-camera format (check your camera manual). Unless the card somehow became corrupted, that should fix it.
If this does not work, and you don't have time to call tech support, you may have to buy a new card if you absolutely must have it for tomorrow.
If you end up getting a new card, make sure to format it
in camera before taking any photos.
Related reading:
Formatting and erasing memory cards
http://www.digicamhelp.com/accessories/memory-cards/erasing.php
