1991 Chevy Silverado will not start

Are there any applicable trouble codes? You should be able to check mil codes yourself. How long since a tune-up? When you get into no code diagnostics, the list of possibilities is about as long as your arm. When the problem occurs, if it cranks good but won't start, visually check for spark at the plugs. If you have spark everywhere, use a gage and check fuel pressure. Some of the national brand auto stores used to loan out a fuel pressure gage, if you don't have one. I can't go by what the shop said or didn't say, I'd have to do my own testing, draw my own conclusions. As of yet, you haven't given me enough info to know where to look?