Hi Therese,
I've had this happen once, and although it was a few years ago, I bet it's still the same. We were never able to figure out a way, even with BB's help, to get that same email sent again. We decided to contact the donor to figure out if a specific fund had been chosen, and printed/screen shot from BBMS for donation backup.
If you go into Online Express to “view transaction history” there is an option to resend the receipt. You can just change the email to your own and resend. You do have to resend one by one, however,