Are you in the UK? As said above, the Citizens Advice Bureau can be of help and advice (at no charge) if you feel you need it. They can help with constructing letters if necessary and if the very worst happens, they can offer financial advice (to a degree) depending on savings, income, health issues etc. The new ESA's (in the UK) are a nighmare series of hoops should you be sick/disabled enough not to work but not so disabled that you cannot work at all..... There's one big UK employer who has no empathy at all for sick employees, so I have an idea what's going on...
I wish you luck.
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me