Is it illegal for my company to dismiss me for "job abandonment" if I am out on approved medical leave?

I have had to have my medical leave extended through work twice (for seizures that I have had to be hospitalized for & tested for almost daily). They seemed to understand for the first few weeks, but after a month they claimed they "didn't receive" the doctor's fax (which they requested.) They had previously asked for my diagnosis and prognosis. I was uncomfortable with this (as I feel exact details should be private - I was on UNPAID leave, after all), but I let my doctor tell them. After they heard I had epilepsy, they stopped contacting me. This is a company in which I have so many close friends (and I have received great reviews for my work.) ADA (Americans with Disabilities Act) covers Epilepsy, but I don't know if I should pursue this. Under 'Job Abandonment' I am not able to claim unemployment! Thanks in advance for your guidance and advice...