You did not specify the type of NEC system, but all of the following applies to the Electra Elite Series of products.
Assuming the the battery on the CPU is good it will hold programming for quite a while. If I remember correctly the manual states that it will hold programming for approximatly 30 days, however I have seen it hold programming for much longer. The only thing you need to check is what Chris suggested. On the CPU card there is a pair of wires that dangles off the front of the card. Make sure this is plugged into the connector on the front. This is connection for the battery.