Often times the cause of overheating is because it simply got pushed too hard (no pun intended) and was used for more than it could handle. It may have run over something as well.
Your lawn tractor could over heat for any number of reasons, the most common could be that the fluid is low. Another thing that you should look out for it is the fuel mix, if it is off engine performance suffers.
The best thing to do is to make sure you have coolant in it. If it does make sure the fans are working and make sure the coolant is running through the system.