Sounds to me lke it is a network error within Windows and nothing to do with AutoCAD. AutoCAD plotting drivers merely access the devices available. If the device is not reported as available, AutoCAD will not plot to it.
If the plotter is in sleep mode and cycling the power works, then it is indeed a problem of the computer, not AutoCAD.
What OS is installed on the system in question?
The reason I ask is that home versions of Windows will fail miserably in a networked environment where there are more than 5 computers installed.
How do you access the network plotter? UNC? IP?
If you access through UNC then the UNC controller could be dropping the older system from the stack, meaning it cannot access network resources. If the state of connection is not set to persist this could be the problem. If you are plotting to IP address, then check to see that there isn't an IP conflict elsewhere on the network. We had a situation once where a computer had a static IP and the plotter had a dynamic IP, but because of the order of boot in the morning, getting the computer with the static IP to plot was a nightmare because the DHCP server would occasionally get the IP address of the static IP system, then an IP conflict would leave the system unable to plot, yet still able to access other network resources.