How can your job terminate you for not being able to return 2 work & do your job duties when u cant

I'm from darlington sc. my doctor took me out of work on sept 26, 2016. I was approved for fmla and short term disability. My job told me that I needed to return to work on January 12, 2017 or they wouldn't be able to hold my job. I explained to them that my doctor still hadn't released me.I wasn't able to fulfill my job duties. ...I had to undergo more test & was hospitalized a few days. They told me that I no longer had a job! I don't understand how you can get terminated for not being able to perform your job duties and is under doctors care. I followed my Drs orders, did my medical regimen as ordered and was finally released february 28. Only thing is I didn't have a job to go back to. They fired me! I put in for unemployment. The unemployment office contacted me and said that my job said I voluntarily quit! I swear to my savior with my hands and feet on the Bible that I didn't. My doctor still has the letter they sent telling me when they expected me to come back! I always kept in contact with Human Resources and the scheduler. I still have the text messages that the scheduler and I text back and forth when she told me my administrator said she couldn't hold my job. My administrator was upset with me because Human Resources told me that I didn't have to tell her every detail that was going in with my health as long as I kept in contact with them which I did. This is down right unfair. How are they able to do such a thing?