It is likely that some type of corruption has happened to your memory card. Try reformatting the card in your camera to correct any problems. If that works then you should be fine to take more pictures from this point. If not, you may need to get another memory card. Check your memory card manufacturer for warranty information.
Just a few tips to help you avoid this problem in the future:
1. Even though your computer can format a memory card, do
not ever do so from your PC. Your computer lays down a
different type of file table than your camera, which can lead
to data corruption.
2. Always format the memory card in the camera it will be
used in after each downloading session AND even when
using a brand new card right out of the box for the first time.
3. Do not share cards among cameras - even if they use the
same type of card - even if they are the same camera model!
Consider your memory card to be camera specific.
4. Do not place your camera close to strong magnetic fields
(e.g on top of a television)
5. Lastly, even though your camera has a single image erase
button, do not do what some do and fill your card up and then
go into the playback mode and erase a few images just to
take a few more new images. This almost certainly will lead
to data corruption on your card. Download your images and
reformat the card to take additional shots.
If you need to recover any lost images from that memory card, you can give it a try by simply downloading free software from the internet (such as Zero Assumption software available at www.z-a-recovery.com
), connecting your memory card to your computer via a card reader, and running the program on the card reader volume.
Hope this helps.