I used to have an 8400, which looks pretty similar, but had a two piece, fold down faceplate... with a one line simple display on the outer, and when I opted to open it, the larger EL display was nearly filling the inner "faceplate"... Now, the reason why I mention this is because it suffered from a similar situation.... eventually, the ribbon cable that connected the outer face and buttons to the innards of the unit would fail and not allow basically anything to work... it would power up, unfold, and do nothing.
Fortunately, the first time this had failed, I was still under the extended warranty I bought with the unit... and when I had it repaired, I talked to the repair shop and found out the part number of the kit that Pioneer makes for this particular repair job. The job from a repair center was about $130-150, and the kit which also came with instructions could be had on ebay for about $15-20 SHIPPED!!
I would highly recommend looking into something like that... because in the life of that unit until I eventually sold it; I had done this exact repair myself about 3 times after it was professionally done... just one little ribbon cable saved me tons of time and lots of money... because I loved that unit and didn't want to spend a bundle on a newer model that did all the same stuff but in a form factor I didn't like as much as what I already had.