Your question is more complicated than it seems. How long were you on leave? Does the company have a written policy about medical leaves. You probably used up your FMLA leave. What was the reason given for saying you were no longer an employee. You should have an attorney look at the facts.
An employer may not refuse to rehire an employee because of a disability or perceived disability. The trick in any kind of disability case is being able to prove the motivating reason for not being rehired is due to discriminatory considerations.
If you can show a position for which you were qualified for was available and the employer passed on you in favor of a less qualified candidate, you are halfway there. The employer must then show a legitimate business reason for selecting that candidate, which was unrelated to your disability.
You should consult with an experienced employment law attorney to review all the facts available to you and to discuss your legal options.