well, interestingly same thing happened to my 8 gb microsd card from sandisk. the mobile would not recognize it, and the mobile would simply hang on accessing the card
as suggested by hanzy, best bet take it back to the dealer. it is serviced by ingram micro, who need the bill number(as told to me by the ingram micro in ahmedabad) under which it was sold by them to the dealer.
in case the dealer is not in the same city(which is what happened in my case), the best you can do is, call him up an get the bill number and with that take it to ingram micro.
the only other thing requried is the orginal box with tag. they didnt ask for the retail bill