i think Hodo was referring to the PTU (Power Transistor Unit).
it was in recall in the earlier Z's. make sure you have a sticker under your hood signifying that you did indeed have this replaced. if not, call the local nissan dealership with your VIN number in hand and they will see if your car qualifies for the recall.
this is a very common problems in the earlier Z's. it's failure acts just like you described. sudden power loss and then it dies, then it will not start again. have the shop that its at test it with a multimeter and see if the OHMs are within spec according to the Z32 bible.
good luck man.