The Canon EOS Rebel T2i is a continuation of Canon's entry-level line of DSLR cameras, the Rebel series. The Rebel T2i is also known as the 550D and the Kiss X4.
I don't have this camera but it sounds like the small slide tab on the memory card may have been moved accidentally when inserting the card or while it was out of the camera. Remove the card and check that the tab is not in the lock position.
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