cd could be "cash declaration", To test, turn the key to x and enter a value into the cash total key then press cash again.
failing that, turn the key to sp and power the till down then, hold down the third button up in each of the first three columns on the left while turning the power back on [same row as buttons 4,5 & 6.
you need to press quite firmly as it's difficult to hit all three keys together]
if successful, the till sends you the all clear message [[ir done] and beeps 3 times, pause beeps once.
sign back on in register, should be up and running