I am wondering how to get the previous version of cards? Can we check the previous versions and restore the previous version?
You can check what previous versions looked like by viewing the card history (in the wrench menu)., but we don't have the capability of rolling back to previous versions (except to examine what a prior version looked like and rebuild it that way from the current version).
Rollback would be an awesome feature, though. See if it's in the Ideas Exchange and enter a new idea if it isn't there already.