Depends what sort of ISA it was, mini cash ISA should in theory be OK*, if it was a share ISA..... ooops.
If it's a cash ISA I wouldn't worry too much at this point.
* I say in theory because it depends how much is in there if the bank holding them went bust you would only get back the first �35k.... and before anybody says UK banks won't go bust I bet you would have said the same about 3 of the biggest investment banks in the world (Bear Sterns, Lehman, Merril Lynch).