There are several things that could be wrong:
1. The VFD or controller went bad.
2. Something on the netplex bus is causing the failure.
3. Something on the mpu got fried.
1. there's not way to test this unless you can swap the display and controller with a good known working unit. (if it still doesn't work, then it is unlikely this is the cause)
2. for the netplex bus, you would start by unplugging everything netplex related except the display (validator, printer, progressive display). If it comes up, then you start plugging in each unit until you find the fault.
3. Pull out mpu board and visually inspect for anything that looks damaged.
It is important to note that at NO TIME should you plug or unplug anything while the machine is powered up, you WILL fry other things if you do so. So everything you are going to pull something out for testing, always do it with power off.