How cold were the temperatures wheyou tried to start it? If it was just below frezing, there"a a good chance you have contaminated fuel and either the fuel filter is frozen or the the condensate in the bottom of the tank froze at the tank outlet. Either case will restrict the fuel flow and prevent starting and running. It"s vey common to have water in the fuel from normal condensation in the tank. Keeping the tank full can help reduce the ammount of water in the fuel but can"t eliminate it. If you are near an electrical outlet, you can use ann electric hair dryer or a heat gun to warm up the filter and drain the water out of it. Better yet replace the filter. If you"re going to do that, there is a product called Diesel 911 that comes in an orange bottle. Fill the filter with 911 and pour the rest in the tank. Follow the instructions on the bottle.I"ve never tried it but folks that have says it works.
Bleed the system per the owners manual instructions. Charge the battery if it"s run down and try to start it.
Cold weather starting is real problem area for diesels because of the nature of the way they work(compression ignition). Starting aids( glow plugs, Thermostart, block heaters)are usually needed because cold temperatre decrease battery output and increases the torque requirements for starting. Then ther are problems with fuel system icing and gelling. Gelling is generally a problem whe someone runs summer fuel in r temperatures below about 15°F or the temperatures are so cold (-40°F)that that even winter blend precipitate parafins out ofthe fuel and clog the filters and the tank outlet.
Icing can happen with temperatures below 32°F which are more common. Water contamination of diesel fuel is common due to condensation in the tank so the combination of freezing temps and the presence of water make for a problem.
Here"s what I recommend. Try to keep the tank full to reduce condensate. Change your filter in the fall before freezing tempertuares occur. Dirty filters cog sooner than clean ones. Make sure the tank has winter fuel in it before winter or add 1 part#1 Diesel to 2 parts #2 diesel to lower the cloud point of the fuel. Use an additive( I use PS White or Stanadyne)that has a water dispersant in it. This will hold the ater nsuspension and keep it from freezing at the bottom of the tank. (If it freezes at the filter you can change the filter.) These additives also lower the cloud point and raise the cetane number a bit to help starting. keep a spare filter on hand just in case you have a problem. make sure that your charging system is working properly. Finally, know how to bleed the fuel system so if you have to change the filter you can quickly do that and get running again.
This is kinda" long winded but I hope it explains the nature of the problem and helps you and your friend to get the tractor going again.