Advice what to do

My project had less people as one resigned and until they hire one I have to push much.. have been working more than 12 hrs past 3 months. Now I'm facing severe neck pain , hr is like I didn't stay long hours in office. I used to work late night to talk to US clients.. I'm completely like broken now that I did all my work which is useless.. Can I get one month break ..will they approve for neck pain or spondylitis. ..I can work but if I start working calls are more than 9hrs ..which is straining a lot.. And I'm so much underpaid, with 15y experience, getting 12l. Not getting time to prepare due to office work.. How to handle situation