Can I be fired on medical leave when following my supervisor's directions?

Hello,
I currently work in Philadelphia for a major airline (greater than 50 people) and for more than 2.5 years (more than 1250 hours). I am a diabetic and recently had a wound on my toe. I went to the doctor who wanted me off of my feet and as a result out of work for approximately 1 month. I obtained the necessary documentation and notified my supervisor and with his help was sending the paperwork to, what I thought was, the right department. Well, find out that I was not sending the information to the right department and as a result the job just thought that I wasn't coming and fired me. Is there anything that I can do about that? Can I get my job back?

You normally cannot just send a letter and not show?

They do have to approve.