If you are not familiar with trouble shooting a failed engine, your best bet is to take it to a qualified shop for diagnostics. You may spend a lot of money blindly replacing good sensors.

Before proceeding, do not use the starter until mechanical checks have been made. More about this later.

I'm curious as to why you took off the serpentine belt. Did you suspect
that an accessory had jammed? 1999 Chevy 5.7 liter, probably a V8.
Engines of that era did not have as great of complication as today's
engines. That should help with trouble-shooting.

Lets start with
belt. It dives a power steering pump, water pump maybe with a cooling
fan with viscous clutch, air conditioner compressor (if present), and
alternator. If any of these jammed, it could stop the engine, but if at
speed, most likely would ruin the belt. When you had the belt off,
were all items free to spin? If so, we can eliminate these.

This leave 3 items: fuel, ignition, or mechanical breakage.

I fear that you may have mechanical damage inside the engine. if that
is the case, great care must be exercised when trying to rotate the
engine. That should be done by hand, and not by the starter. The first
step is to determine if there is any interference or non-moving parts that should move inside the engine. Label the spark plug leads and pull all plugs. Disconnect the power supply lead to the ignition system. With the plugs removed, the engine should be fairly easy to rotate. It may be possible to rotate the enigne by pulling on the belt, but I do not recommend that due to the danger of trapping a finger between the belt and sheave. A safer way is to apply a socket wrench to the damper nut. If you have a distributor, remove the cap to see if its rotor moves properly. It should rotate once for every 2 engine rotations. Listen for any unusual noises while rotating the engine. The engine should rotate smoothly but you may feel a little cam lobe action.

This next step requires new valve cover gaskets. It may be possible to reuse the old ones temporarily if these do not tear when removing the valve covers. Remove the valve covers and look for any broken or cracked parts. Rotate the engine by hand and observe the rocker arm actions. Do all work? At this slow speed, the rockers may not show full travel due to leak-down of the lifters.

Lets hope it was not the timing chain. I do not know if your engine is a free turning engine or not. If not, the pistons could have hit the valves - that's expensive.

If you have a roller cam chain, worn ones have been known to jump cogs and if the chain did jump more than a couple of teeth, the engine may quit and will not start. However, I have seen engines continue to run after the chain jumped one tooth. There was a sudden loss of power due ignition retard of about 15 deg and valve timing was off. This engine limped home after advancing the ignition - not recommended. If the chain jumps again, the pistons may hit valves. Toothed timing chains rarely jump teeth, but can get sloppy.

The ignition could have quit. That'll take some diagnostics best done at a qualified garage. If you have a distributor, there is a sensor in it that times the spark. On other GM models of that era, that sensor was a weak item. Without a distributor, spark timing may come from a sensor that reads holes in the flex coupler plate on the backside of the flywheel. These are the simple items. Other things are more complex.

Most engine control computers have a method of checking for engine rotation. If it senses that rotation or ignition has ceased, it shuts off the fuel pump. Sometimes, you can hear this action at start up. turning the ignition switch to the run position activates the fuel pump. If nothing more is done, the fuel pump runs for 3 to 5 seconds and stops. Sometimes, this can be heard. A fault in this system can disable the fuel pump.

If your battery is not fully charged, for example, the charging system had failed, the ignition will quit when the voltage falls below a certain voltage. The next step is not a full test, but will indicate if the battery is low. Put a volt meter across the battery terminals to read voltage. At no load, nad fully charge, it should read a bit more than 12 V. Turn on the headlamps on and read the volts, then change to high beams and read again. The battery should be able to supply these lamps with much voltage drop. If you see the volts drop to 10, the battery is insufficient. (This assumes you have a 12 v system.) A deficient battery explains why it can not turn over the engine with authority.