Can't insert compact flash into canon rebel xti

Question

i was shooting at night and switched my cf card. i must have accidentally pushed something... because i couldn't get it in! nothing looked out of place. when i shut the door and turn it on, the display said there was something wrong with the card (even though there was no card) and to put a new card in. i am a helpdesk tech and i am completely stymied. help help help!!!!!!!