as someone else said, multiple pregnacies are much more common now because we have early ultrasounds, which they used to not do. many women never knew they were originally having twins, and went on to have one baby. there is no reason to think your remaining baby wont be perfectly fine.
i hope everything works out for you. at this point, wait for your next ultrasound to make sure the results are the same. there is always a chance for mistakes at 6 1/2 weeks. however, if the results are the same, then grieve your lost baby, but remember you still have a little one coming.
in the middle or your worry, dont forget to enjoy the wonderful site of that little heart beating you did see. good luck to you and your family