Advice please

I sent my medical questionnaire back two weeks ago, I received a letter today saying that I have been placed in the Work Related group, it says if i'm not happy I can write and ask for the decision to be looked at again and for an explanation how they made their decision.
Would just writing a letter be okay and should I explain why I believe that I'm unable to return to work????
I'm completely confused any advice would be welcome
Many Thanks

Thank You for the reply, I wouldn't have known how the fill the questionnaire in without this website

I really don't know what to do now, I'm worried if I ask for my form to be relooked at that I could lose my award.

I said on my form that I can only go out when accompanied by someone, I don't cook because I leave pans on,I have no idea how i havent burnt my house down . I have to arrange something weeks in advance to be able to cope with situations, I cannot be around new people and I've been known to get really angry in public due to my depression. I was waiting to be referred to a psychiatrist when I filled in my form and now after my appointment my medication has been changed and one has been doubled.

I'm now worried about what happens in a year when my benefit is stopped, and how this new process will work