There are individual fuses for the power supply for each thruster, there are also fuses for the communications lines to each thruster. Any one of these could be blown, they are located on the main PCB in the pod on the vehicle.
Or its the thruster itself that is gone. I take it you mean when you display the list of vehicle nodes with serial numbers, the port Fwd is blank. Also try doing a Node search to see if it picks it up. Most likely a fuse tho. Be careful changing the fuses out, the surface mount holdings are quite flimsy for the comms line fuses.
For the power fuses, there is a Self test button on the board that shows if all fuses are OK at the press of a button.