Sierra Classic won't start

My son took my truck to conduct some business. He drove approximately 7 miles, took care of business, went back to the truck, and it wouldn't start. It cranks, sounds like it's about to turn over, but won't. My code scanner says 12, but then nothing.
After an hour of trying to start it, with the help of some mechanics (VW though) we finally had it towed home. Can't figure where to start looking for a problem. Any ideas?

What year, make, model, engine size?
Did you check spark,fuel pressure?

I'm so sorry! I had just finished posting my auto info on the registration, and just didn't think about posting it here! DUH!

Ok, it's an '88 Sierra Classic. Rebuilt motor, 3 1/2 years old. rebuilt trans, 3 yrs old, 8 cylinder.

I can check the sparks today, don't know how to check the fuel pressure. It does sound as if you just need to give it a bit of gas and it would turn over, though.

rh

spray some starting fluid into throttle body and see if it fires if not check spark

Thanks for the suggestion. We got it started and it ran for a couple of days and a hundred miles before it died again. Replaced the fuel pump, and all is well.