Could not travel because of an illness

[venuserena]

I booked a flight last February, but several days before the actual flight (in May) I felt extreme pain and the doctors discovered stones in my kidneys. I was strongly advised by my treating physician to postpone the trip to Europe. I did so, talked to the Delta customer service, and canceled the reservation.
Because I am a graduate student, I can't travel during the academic year (August - May) and asked Delta (in an email through their official website) to allow me to use the credit I have with them (over 1,200 US dollars) in the summer, when I will be able to use it for a trip to Europe (where I live) or for research purposes. I explained that as an international student this amount is quite large and equals to almost two times my monthly rent.
I just received an email from the Customer Service notifying me that my request has been denied. I cannot afford to lose 1,200 dollars, but I also can't travel before February because I am preparing for my PhD exams and have obligations to the department that require my presence.
Please advise me what to do and if there is any solution that you might think of.
Thank you in advance!