It is unlikely that you are doing anything wrong. If it is the same card as before, it is likely your card may be faulty. If it is a different card, it is likely your 7D may be faulty. If you are able to access the card from a card reader on your PC, then the card reader in the 7D may be faulty and if you can not, then it is likely the card has become corrupt and you will need to try using a file recovery tool such as the ones described in the answer that Philip Kendall linked toanswer that Philip Kendall linked to.