Did you follow the instructions given by koa:
I had a DX21 with black squares but the battery was fine. The display would come back to normal when would reload the sounds by pressing Character Function (Compare on the dx27), then 12, then Yes to confirm.
This should normally bring your DX back to life...
A weak battery affects the sounds stored to the RAM. So you have to reload them (by using a RAM/ROM card or via MIDI (sysex file). Then store it to the internal memory, enable the internal memory protection... and all should be fine...
My (key related) gear (in alphabetical order):
Kawai: K1 II
Korg: M1 (up'd to EX), M1REX, Poly-800
Roland: D-50, D-110, D-550, PG-1000
Yamaha: AW1600, DX7IID, EX5R, RM50, RX5, SY99 (w. 'Cool Blue' mod), TG500, TX-802, YMM2
Sector101: SYEMB05 (5x), SYEMB06 (3x), EXFLM2 (1kit), MCD Sweet16 (1x)