Perhaps the memory card is corrupt. Try using another memory card, and always formatting them in the camera before use. (note this erases all pictures from the card.)

If this doesn't work, try using the camera's reset function. I believe this is done by holding down two particular buttons, which may be marked with green dots. If not, check the manual.

Also try taking out the battery, waiting 5 mins and replacing it.

If the lens has an aperture ring, make sure it is set to the smallest aperture (biggest number).

Try a different lens to see if it's the camera or lens.

Otherwise I fear it may be a repair job. :(