I have a John Deere riding lawn tractor model LX178. I was riding it down the gravel driveway when I hit a rock and the blade belt came out of the housing. I do not have a diagram on how to wind it back in place. Would anyone be able to give me instructions on how to fix it as I need to cut the grass before a forest grows! Thanks.

First off, if you hit a rock that hard you likely have a damaged blade or mandrel so inspect carefully. What model do you have? john deere belt diagram - Google Image Search

Hi,
Generally a LX178 has a 38" deck, but there are two of them. The material for the spindle housings is different, aluminum with four bolts and cast iron with three bolts.
Deere has parts lookup by picking Residential, Service & Support and Parts; should take you to JDParts and you can enter your model in the box at the upper left.
Peace,
Clarke