Netplex is simply a two-way communications protocol the CPU uses to talk to devices installed in a machine. There are a number of things that can cause this error. The most common, and the first thing to check, is the power supply which is located behind the reels. Be sure it is well seated and not loose. You might try unplugging and reseating it. If you have another S2000 machine, try swapping power supplies and see what happens. If you are unable to swap power supplies, measure the voltages. There should be 13vdc and 25vdc output.
This is at least a good starting point. When you get this far, this forum will help you with the next steps on things to do.