I hate to say this, but...restore from your last backup.
When you change a datatype so radically, it doesn't try to parse them, it basically throws it away and starts again, because there is no guaranteed conversion from a varchar "hello" to a valid date.
When you have restored the old version, follow the advice here:
http://stackoverflow.com/questions/7090231/how-to-convert-a-varchar-column-type-to-date-type-without-losing-the-dates[
^]
But if you don't have a backup, you are pretty much stuffed - it's a complicated change to undo, if it is even possible.