Parameters defining unauthorized absence in company

I am working as an Executive in a private sector bank. I visited USA on 30 days sanctioned leave. I extended my leave by 38 days producing medical certificate; received a communication from the Bank that the leave has not been considered favourably and advising me to report to duty immediately. I informed the Management that as I am under treatment and has been advised against travelling, it would not be possible to report to duty immediately. After joining duty, the Management has treated my extension of leave period as unauthorized absence. My question is: a) whether the Management can treat my application for sick leave as unauthorized absence? b) what are the parameters which determine whether an employee was on unauthorized absence? c) are there any decided cases similar to my case? d) what is the remedy available to me (to get unauthorized absence converted as sick leave)? Thanks in advance.