1998 Chevrolet Silverado 1500 will not start after rain ?

Won't turn over means the engine will not crank [ no movement]...won't start means the engine crank,s [ has rotational movement] but will not start running. ..I'm assuming you mean it crank,s but won't start running. Your problem is either fuel related,ignition related, or mechanical related[ as in timing chain problem. Try this....open air intake...between the air filter & the intake manifold...spray a small quantity of carburetor/injector cleaner towards the manifold,close the intake & try to start...if it start,s then die,s..it,s a fuel delivery problem. if not...then check for spark at the spark plug,s [ Remove a spark plug, ground the base on the engine,connect the plug wire,then have someone crank the engine while you observe the plug for spark.] if no spark then it,s an ignition problem...if there is spark then check your engine timing or on board computer...have the computer code,s checked...there are many computer sensor,s that can cause your symptom,s...without hearing what,s going on it,s difficult to diagnosis but if you suspect water contamination..have the ignition system checked out. if it,s water then have the system waterproofed with ignition sealer.

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Most likely it's a problem with Moisture getting into the Electrical or Ignition system, and disrupting the Current flow.
Try spraying WD-40 on the Connections around the Distributor, and the Wiring Harness, and it will probably Start.
If that Solves your Immediate Problem, then you know the Culprit, and can take steps to prevent the Problem in the Future by using Dia-Electric Grease on all the Connetions to help keep the Moisture from getting in.
You should also look to see if water is leaking into the Engine Compartment through an opening such a Hood Scoop, or Vent and where it is collecting on the Engine or Firewall, and what Electrical Connections or Componets are being affected, then Sheild them, or move them if possible so they won't get wet.