No way to diagnose network problems verbally but ive had several.
Windows updates on either machine can sometimes cause the saved connections to be lost, or stop functioning. Can be due to network driver changes, or antivirus changes or windows system changes. The fix is usually to start over with network setup on all machines. I also find that the QB connections are more reliable when all the connected machines are on the same version of the same OS.
Don't overlook physical problems: cables that have been run over by a chair or have loosened in the socket can stop working even if they appear to be plugged in.
