Welcome to the forums!
When it did not start were there any engine lights on dash etc?
Have you scanned for any history/stored codes?
Generally the place to start is from the battery, load test it to ensure it is good.
Next make sure all the battery cables/connections are clean, free from corrosion & tight.
The security may also cause issues like this as well as ignition.
You can Google 2003 Silverado intermittent start, you are not alone in problems unfortunately.