Why would a 1989 Chevy Blazer not start after being washed?

Did you wash the engine or just the outside?????????????????? If you washed the engine and it was hot when you washed it, you possibly got water in one of the electrical connections. Never ever wash an engine with sensors when it is hot. The water turns to steam and the steam gets in the connections . You can wash a cold engine bay just try not to direct high pressure into areas like the computer and sensors. If all you did is wash the outside.....I do not have a clue what to tell you. You may just need to wait until things dry out