Facing Surgery and a Warning Letter: Can I Challenge My Employer's Decision?

I'm working in a very big retail company, and I'm currently in a probation period. Two months ago, I met with an accident and injured my knee. Last Sunday, I experienced severe pain, so I went to the doctor for a check-up. He recommended an MRI scan, and based on the scan report, I need to undergo surgery. Consequently, I am unable to go to work, which I informed all the higher authorities about via SMS. Despite this, they sent a warning letter to my home by speed post.

Seeking Advice on Challenging a Warning Letter

Please advise me on whether I can challenge this letter.

Thanks & Regards,
Atul

Instead of making any mistakes this time, please send them your medical documents either by mail or in person (through someone else) so they can be certain of your situation. They only require valid proof of the reasons for your leaves.

Please handle this first and then observe the outcome...

Thank you.