I just had a second hair transplant; it appears that a significant number of previously grated hairs fell out?

You can sometimes damage exiting hair during a hair transplant (including hairs from prior hair transplant).  It is rare.  If it is an issue you should see your doctor.

Yes, this can occur.  Fortunately, it does not happen often, but it can occur.  Just hang in there.  All the older grafts along with the new grafts should eventually make their way back in and you should have better coverage than before your second transplant.  It's never easy when this sort of thing happens to you.  

The hair fall of previously grafted hairs which you have experienced can occur after Hair Transplantation in the overlapping zone on the first and second Hair Transplant. There is nothing to worry as these as well as the new hairs will be come back in 3 to 4 months.

Do not worry, about 3 weeks after your procedure, it is the norm for follicles, transplanted and non transplanted, to shed and regrow at months 3-6. 

They may grow back. This may be shock loss or telogen effluvium and can be reversible in some casses.

Though it is not the norm, personally I think this problem is a lot more common than we surgeons would like to admit. (It was actually my experience following my own 2nd HT.) Fear not: it will start growing back in 3-4 months. The combination of finasteride and minoxidil may help.

I have been in practice now for 25 years and have seen this happen in about 6 patients. When this did happen, all of the previous grafts came back when the new transplants grew. Considering that my group has performed over 15000 surgeries, this is rare. More common is shock loss in the original hair which can be prevented with the drug finasteride.