Draw a map!
Yes that’s right, the golden rule of network troubleshooting is to draw yourself a diagram. How do the computers, switches, and routers plug in with each other on the network. Are you running VLANs, how is traffic supposed to flow from point A to point B. Put it all down, you don’t need to use Visio a napkin will work in a pinch.
After you have your map drawn, validate it, what happens when you traceroute/ping from point A to point B? Does the network follow the path you think it should? Where does it deviate from the path? Go to that device and traceroute/ping, are the results different? Inspect the routing table, where does the system think it needs to forward traffic to?
If you are thinking to yourself that this seems really basic it is, but I guarantee that more often than not this will help you track down a problem.