The Stars Must Wait — Keith Laumer

In the year 2002 a young Air Force Lieutenant Commander prepares for his role as Backup Navigator for the historic Callisto Mission. Before leaving Earth for space, "Jack" Jackson will be placed in a state of suspended animation, and will be awakened on arrival at Callistounless something happens to the Primary Navigator....

And Jackson slept on. And on. When he does awaken, a century has passed, and he is faced with a nightmare wilderness inhabited by neo-barbarians and senile--but still sentient--tanks. Now it's up to Jackson to bring the world back to its senses.