I think the 2000 altimas have coil-on-plug ignition.

Anyways, if you are getting spark, fuel pressure, and the motor is turning over, then maybe the immobilizer somehow got tripped and the ecu is not sending the (ground) pulses to operate the injectors. Check for the clicking noise of the injector by holding the tip of a screwdriver to the injector and putting your ear up to the handle. If no clicking, and the injectors are recieving +power, then the ecu is it, or the immobilizer. Re-program the ecu and keys (nissan dealer) if it is the immobilizer.