1st and 2nd early potatoes only keep for a few weeks, in fact it is better to just lift the amount you need on the day you need them. They also need to be stored somewhere cool, dry and with not a lot of light. Where ddi you put the sack and was it exposed to light?

I have just answered your Q on Maris Peer and would echo the advice given by MG. You cannot eat green potatoes they are poisonous. If you can, cut the green away, but if it is widespread throw them out. You could retard the greening by burying them in the ground again. Cover over with sacks to keep the rain off. That way you can take out what you need day to day. Greening only occurs when the light is getting to the potatoes.