Hello I have 1998 nissan altima and my question is, what would cause me to lose all electrical power? No dash lights and wont crank.

SOURCE: How do I remove a starter off a 1998 nissan Altima?

need an engine size

SOURCE: CRANK SHAFT POSITION SENSOR 20002 NISSAN ALTIMA

1st, Nissan had a recall on Crank position sensors for 2002-2006 Altima's. Check with you dealer before you go doing any repairs. You may discover you can get your car repaired at no charge.
NHTSA Campaign ID number :O7V527000
Symptom: Erratic behavior. Stalling and no starts and very difficult to diagnose due to the time frame in which the component actually malfunctions. The warranty should not be limited to the original owner.
I would also need more specifics as to which engine you have so I might assist you in where the location of the sensor is. The Altima had a 4 and a 6 cyl engine offered that year.

SOURCE: high idle stalls at lights code for crank sensor

Crankshaft and Camshaft Position Sensors Camshaft Position Sensor Engine timing is determined from the relationship between the crankshaft and camshaft. This relationship is maintained by a timing chain or timing belt. The Engine Management System does not control engine timing but it does monitor the relative position and speed of these shafts by monitoring signals generated by sensors. The Engine Management System (EMS) uses signals generated by the camshaft position sensor to synchronize fuel injection to the valve sequence and for the on-board diagnostic procedure for misfire detection. The EMS energizes the injector at or near the time the intake valve opens. For misfire diagnosis, the EMS compares the number of camshaft sensor reference pulses and the number of crankshaft position sensor reference pulses received. If the EMS receives an incorrect number of pulses Diagnostic Trouble Codes (DTC’s) should be stored in the EMS.
If the camshaft position sensor or circuit is faulty most engines will start. Some engine management systems will then default to a pre-programmed injector firing sequence. All injectors may be energized simultaneously or all of the injectors on one bank may be energized at the same time. If the CMP data is required for misfire detection and reliable CMP data is not present misfire detection would probably be suspended.
Related Symptoms The following symptoms can be caused by an intermittent wiring connection or faulty signal to the EMS:

• Extended crank time with a cold engine
• Intermittent rough running
• Unstable idle
• Bucking
• Hesitation
• Stumble
• Chuggle
• Poor fuel economy
• Stalling on acceleration

Crankshaft Position Sensor Engine speed is a very important input to the Engine Management System (EMS). Crankshaft speed and position are the basis for many calculations made by the computer. Crankshaft position values are transmitted to the computer by pickup coils also known as Permanent Magnet (P/M) generators, hall-effect sensors or optical sensors. The Crankshaft Position Sensor (CKP) also known as engine speed sensor is located in close proximity to the crankshaft.
In addition the EMS uses minute variations in the CKP sensor data to determine engine misfire. The EMS uses this information in conjunction with the camshaft position sensor to perform misfire diagnostics.
Related Symptoms: No Start/Intermittent Start Condition – Can be caused by a faulty crankshaft position sensor due to loose connections, bad grounds, high resistance in the circuit, or opens in the circuit

SOURCE: 2002 Nissan Sentra Loses Power

SERVICE IT .....OIL PLUGS, AIR ,FUEL AND OIL FILTERS GET TROTTLE SENCOR AND THE MASS AIR FLOW SENCOR CHECKED

SOURCE: MY NISSAN ALTIMA 2006 2.5

did you check the sensors and if you are getting fuel and spark