I downloaded some pictures to my school computer and now my camera won't take any pictures. Message is Memory card write protected.

SD cards have a slide switch along one edge. The position farthest from the contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card. If the switch is already in the proper position, slide it fully the other way and then back again.

If that doesn't work, try another card.

Hello,

Could perhaps the Disable Delete on the Set-Up menu have been set?

Here is what the manual says...

Protecting images from Deletion: Disable Delete
The Disable Delete option is used to protect images
from accidental deletion. Select On to disable
all operations or menu options that delete or
erase images, including deletion with the button,
the Delete option in the playback menu, and
the CF Card Format option in the setup menu.
When Off is selected, images can be deleted and
memory cards formatted normally.

Hope this helps.