DLA renewal

I am in blind panic! this is the first time I have had to renew my DLA higher mobility allowance. I had no doubt that I would pass any tests as I am crippled with osteoarthritis in my knees (one replaced but not very successful) hips, base of my spine, hands and right elbow and shoulder. I have chronic lung disease (industrial, I have never smoked) and I have had several TIAs or mini strokes which badly effect my memory. I returned my completed renewal forms on the 3rd July which I thought was in plenty of time for late September, I could not believe my luck when I did not receive an invitation to appear before the inquisition! and thought it was probably because of the change to PIP which was coming up. My wife went to collect my DLA a couple of weeks ago and there was a reduced amount, I tried several times to e-mail the DWP because I am still not very confident on the phone but I had to phone in the end. I went through all the questions with the help of my wife (on speakerphone) then the lady at DWP just casually said your claim has ended! I was stunned and asked why to be told that I had not returned my forms, I said of course I did, my wife posted them and I marked the date on the calendar she said can you prove it!!! I said what do I do now and the reply was "there is nothing you can do, your claim is finished and you are too old at 66 to start a new one) I said why would I not return my forms, this money is crucial to me. It means the difference between keeping and loosing our home as I will be unable to keep up my mortgage and I just don't know what to do.