I would certainly agree with the others that it is probably the fuel filter. I fully service my tractor once a year. When I buy new fluids and filters, I always buy at least one extra fuel filter -- so that I have two extras in the workshop after I service.
This is the second year in a row when I have experienced the same basic problem as you describe. Both times it has happened when the temperature has dipped down to about 10 degrees F. The filter basically puckers up and collapses with mushy ice and dirt. I find it truly disgusting when I pull it out. It will be black and slimy. Putting in a new filter gets the tractor running immediately.
This time of year, it doesn't hurt to put some diesel fuel additive in the tank either -- like cetane boost with water absorber and cleaner -- especially if you are using fuel you bought in during the summer. It will help it keep from jelling, and it will absorb some amount of moisture that can form in the tank from condensation.