Notice Period Issue with HR and PMO

Hi all, I am serving NP, I am going to office continuously, But for the last 10 days, I didn't go due to health issues, I already had this condition and asked WFO exception from my Manager, both Hr and manager rejected it. So I went for some days, and now didn't go. Last week my manager called me and threatened me with extension, even though I have vaild medical documents. Now the HR has asked me for a discussion regarding this. What should I do now?What are the possible consequences. Shall I raise grievance against my Manager?