When you say "won't turn over", to some of us that means the engine won't rotate. The other case would be it turns over but won't start.
If it won't start, follow lasers advice and if you do have spark, remove the air cleaner, make sure the choke is closing and try squirting a small amount of gas into the carb.
If it doen't turn over, then try it with a fully charged battery. No go take a small wire attach it to the battery positive and the other side to the small post on the starter relay. If there are 4 posts on the relay, pick the small one with the colored wire. You should get either the starter to turn or a good click.
If the starter turns then you have a safety switch issue, blown fuse, ignition switch or bad connection some place.
If it just clicks, it's either the starter gear is locked into the flywheel, the starter is bad or the solenoid (relay) is bad.
Narrow it down for us and we can send you down the right path.